1. In terms of silencing a gene:

• RNAi: it 'knocks-down' a gene. Meaning: cells might still have some gene expression and contain the transcripts of the GOI (gene of interest).
• CRISPR/Cas: it 'knocks-out' a gene. A null allele can be generated, resulting in no gene expression at all for the GOI.

So, if you want to completely silence a gene, CRISPR technology is better.

2. CRISPR/Cas9 technology have many other applications, which RNAi technology doesn't have. For example, CRISPR/Cas9 technology can substitute bps of an allele, fix/repair an allele, replace an allele, help to integrate a gene at a specific locus, perform CRISPR-mediated in vivo real-time imaging,....etc.

The previous answer is correct but if I may add, RNAi will generate multiple lines each silenced at a different level. Thus you have to sreen many lines (by Q-RT-PCR, Northern or western) and when doing experiments reviewer woudl ask you to do the experiment with several silenced lines if you want to publish in good journals.

On the other hand RNAi is super simple and protocol for RNAi may be easier to find than for CRISPR

Essential genes

Some genes are essential. It may be lethal to the plants if you knock out the genes (by CRISPR/Cas9). In this case, RNAi-based technique is a better choose. From different degree of suppression of gene expression of GOI using RNAi (the transgenic plants are still alive), researchers can peep into the function of the specific gene.

Hi

It depends on your goal. If you want to reduce the expression, RNAi is better. But if you want to turn off  the gene entirely, CRISPR is better. However, this method requires high skill and expertise

Which one is better?

Yes, indeed, it depends on your goal of your researches (@ Fahimeh Mohammadi). They can be equally good, and the combined use of data from both method can improve research results. One example is from the paper published on Nature Biotechnology1. The authors compare CRISPR/Cas9 and RNAi screens for essential genes using a human cell line.

The authors found out: "the precision of the two libraries in detecting essential genes was similar and that combining data from both screens improved performance." 

1. Paper Title: "Systematic comparison of CRISPR/Cas9 and RNAi screens for essential genes"  [http://www.nature.com/nbt/journal/v34/n6/full/nbt.3567.html]

A good review paper for you, although it based on mammalian cell researches.

"RNA Interference in the Age of CRISPR: Will CRISPR Interfere with RNAi?"

If you want to edit the genome then CRISPR is the best choice but be sure that this editing is not lethal for cells.Good luck.

Update:

• Recent results (using CRISPR/Cas9) published in eLife have raised questions about the validity of previous screens which have predominantly used RNAi.  (https://www.ncbi.nlm.nih.gov/pubmed/28337968)
• Heidi Ledford, Nature News, 05 April 2017, http://www.nature.com/news/crispr-studies-muddy-results-of-older-gene-research-1.21763

CRISPR is also capable to be applied for regulation of transcription, and in that case, the technique is termed CRISPRi.  There are several articles on CRISPRi in animal and bacteria models, showing good reduction in transcription activity.

You might be interested in this paper https://www.ncbi.nlm.nih.gov/pubmed/26168398 

Nature. 2015 Aug 13;524(7564):230-3. doi: 10.1038/nature14580. Epub 2015 Jul 13.

Genetic compensation induced by deleterious mutations but not gene knockdowns.

Rossi A1, Kontarakis Z1, Gerri C1, Nolte H1, Hölper S1, Krüger M1, Stainier DY1.

In the age of generating CRISPR/Cas9-mediated gene knockouts, amiRs are still desirable as they create knockdowns, which can be useful for gene functional studies in cases where null alleles might be lethal. Moreover, it may be challenging to knockdown multiple genes of a family simultaneously, using CRISPR, but those can be achieved relatively easily using amiRs. CRISPR-mediated knockouts are permanent, while amiRs can transiently knockdown target genes, apart from creating stable transgenics.

Things to take note of with CRISPR are adaptation, clonal heterogeneity, copy number effect, and limited sensitivity.

For RNAi, the major drawback is off-target effects producing false positives, which siPOOLs (https://www.sitoolsbiotech.com/sipools.php) help to counter.

Both techniques however are inherently complementary as one is a knock-out, the other a knock-down. Best used together as recommended for full evaluation of gene function!

For useful articles on CRISPR and RNAi, please visit http://blog.sitoolsbiotech.com/

THANKS CATHERINE

CRISPR and RNAi are both powerful tools used for gene silencing, but they operate differently and have distinct applications:

1. Mechanism of Action: CRISPR/Cas9 is a genome editing tool that can create double-strand breaks at specific locations in the DNA, leading to gene knockout or allowing for gene editing. RNA interference (RNAi), on the other hand, involves the silencing of gene expression post-transcriptionally by degrading mRNA or inhibiting its translation.

2. Permanence: CRISPR/Cas9 modifications are permanent and heritable as they alter the DNA sequence. RNAi effects are temporary and reversible since they do not change the underlying DNA sequence.

3. Specificity: CRISPR/Cas9 can be highly specific when carefully designed, although off-target effects can occur. RNAi can also suffer from off-target effects, particularly due to partial sequence homology with non-target mRNAs.

4. Efficiency: CRISPR/Cas9 can completely knock out gene function, while RNAi typically reduces gene expression to varying degrees, which may not result in a complete loss of function.

5. Delivery: Both require delivery systems into cells; CRISPR/Cas9 components are typically delivered via plasmids, viruses, or ribonucleoprotein complexes, while RNAi involves the delivery of small interfering RNAs (siRNAs) or short hairpin RNAs (shRNAs).

6. Development Stage: CRISPR/Cas9 is a newer technology and has rapidly evolved in recent years. RNAi has been used longer and has a more established track record, especially in therapeutic contexts.

7. Applications: CRISPR/Cas9 is widely used for creating gene knockouts in model organisms, gene therapy, and functional genomics. RNAi is often used for gene function studies, therapeutics, and is sometimes preferred for transient gene knockdown experiments.

In conclusion, the choice between CRISPR and RNAi depends on the goals of the experiment, the required duration of gene silencing, and the organism or system being studied.

Perhaps this protocol list can give us more information to help solve the problem.