If you have an LS it's easier to stick a VTEC head on the block you already have. Putting in an entire GSR motor is a bit more involved. Also pretty expensive in comparison to just buying a VTEC head and slappin it on an LS block.

From an engine design standpoint the LS has a longer stroke which gets you more power lower in the revs. The block isn't really built to take the higher revs so if you truly want it to be reliable with extended high-RPM running you need to do some beefing up.