SR-25 and MSG-90 are relatively the same except the SR-25 has a faster Rate of fire and the MSG-90 does slightly more damage.
With that said, the SR-25 is much more practical. Either way both guns are a 2 shot kill and so doing more damage doesn't really make a difference. The idea around a faster rate of fire of the SR-25 is that you'll be able to get the killing shot off much quicker than the MSG-90 (After taking the first hit with the MSG-90, your target has a good half a second to react by ducking behind cover; SR-25 has about a quarter of a second pause between shots)
Conclusion: SR-25 > MSG-90