Samsung Galaxy S i9000 has a Super AMOLED display which is somewhat superior to the SuperClearLCD on Samsung Galaxy SL i9003.
Samsung Galaxy S at the same time has a slightly higher amount of RAM of 512mb compared to Samsung Galaxy SL having 478mb. Well, not much a differnce here.
Galaxy S have a better Graphics Processor : SGX540 instead of SGX530 on the Galaxy SL.

Galaxy SL is said to be manufactured to fight the shortage of AMOLED panels and reduce the production cost as the price of Galaxy S can't dive any further.

Personal prefernce, I would prefer the Samsung Galaxy S over the Galaxy SL.

At the same time, did you check out Samsung Galaxy S2 which have been confirmed by a forumer that it will be brought in by SME by end of June and he's selling at RM2099. It packs a Dual-Core processor and a superior imaging sensor of 8mpx with Flash. The screen is larger as well and the OS is much newer. Plus, it even supports FullHD video recording. Not to mention the doubled amount of RAM available. Hope this helps.