The difference between miRNA-27a-5p and miRNA-27a-3p is origination of mature micro RNA sequences from precursor micro RNA. 5p and 3p depicts the direction of DNA/RNA strand.
In this case precursor micro RNA for both miRNA-27a-5p and miRNA-27a-3p is same.
To know which one is better from miRNA-27a-5p and miRNA-27a-3p you need to find their target genes using tools such as TargetScan, miRanda, Tarbase etc. On the basis of targets of your micro RNAs you would know which one is better.