Dear Siddhant Singh, in order to get an answer compare their 'standard reduction potential'. Please have a look at the following video. My Regards

https://youtu.be/pbk18t2yDpE

LiBH4 is stronger than NaBH4 reducing agent because of the high degree of dissociation of it as as metal cation Li+and borohydride as a anion-BH4 .

Dear Siddhant Singh, the two reagents have different advantages. The two reagents work by different mechanism. Normally NaBH4 ismuch cheaper and used in protic solvents like methanol, ethanol, aq. NaOH or even water. Here the proton of the protic solvent activates the carbonyl group by protonation followed by hydride ion transfer from borohydride ion, Thus Na+ cation has no role in activating C=O. Sod. borohydride can not be used inaprotic solvents like ether or THF or other common solvents (with which it don;t reacts) because of its poor solubility. But in case of LiBH4 , Li-cation has better coordinationg ability to to coordinate with C=O and it activate C=O making it a better hydride ion acceptor from borohydride. Here Li+ has a distinct role in activating C=O and it can be used in approtic solvents like ether, diglyme, THF etc. However when LiBH4 is used in protic solvents like water Li+ being of smaller size and high charge density is highly solvated and can not coordinate with C=O. In such solvent the proton of protic solvent activates C=O as in case of NaBH4 / MeOH. As LiBH4 is soluble in ether , THF, diglyme it has an extra advantage in using for reactions sensitive to moisture. Normally peoples use LiBH4 over NaBH4 when they need to carry out the reduction in anhydrous condition.

When comparing NaBH4 with LiAlH4, the metal-hydrogen bond of LiAlH4 is more polar; thus, it is a stronger reducing agent than NaBH4. This is mainly because aluminum in LiAlH4 is more electronegative than boron in NaBH4. Below infographic gives a detailed comparison related to the difference between NaBH4 and LiAlH4.