If you compare the both,
SL has 4.0" screen @ 480 x 800 and DEFY has 3.7" @ 480 x 854
Well, DEFY got some more pixels in low screen size so its display surely will look good than Galaxy SL (not much, but negligible difference)
SL has 4GB/16GB internal storage while DEFY comes with 2GB. In case of RAM Defy has 512MB's RAM while SL comes with 478MB's RAM.
Galaxy SL = Bluetooth 3.0 (Futureproof), Defy = still the same B2.1
Both of them have 5mp camera but DEFY got LED Flash that's the good thing about it.
Galaxy SL can record videos @ 720p which is not possible in DEFY. Even SL got secondary camera (Facetime, 3G) which is absent in DEFY. So if you are looking over a futureproof phone then better consider SL.
SL has Froyo ROM, while DEFY still has Eclair ROM (upgrade will be coming soon, maybe). Both of them have Cortex A8 processors only SL got 1GHz clocks and DEFY has 800MHz clocks.
Conclusion: Comparing both, SL looks impressive. Unconsidered the LED Flash and better display of DEFY better go for SL.