Beside the already mentioned differences I think the only points missing are that the slim model has ~1hour longer battery life and feels much cheaper produced then the original version and that the slim model is ~40grams lighter then the first version (without 3G module, it's even more compared to the 3G model).
About the OLED screen you should be aware that it's one of the more early OLEDs which means not only the burn-in is a serious problem if you often play the same game (remind that e.g. HUDs are always at the same location, even if they tend to be translucent). Alot of original Vita screens I saw also already got a yellow tinct (OLED problem).
The OLED of the original looks much more vibrant (actually too vibrant, but I guess it could count as plus for the many cartoon/anime-optic games on the Vita) then the screens of the slim model, but I really only recommand to buy a OLED model if it's brand new or at least not often played due to the screen issues.
You shouldn't care about the 1GB internal memory. Once you installed a memory card (you should get a vita2SD, the prices for Sony's Vita memory cards are a bad joke...) you also only can use it when you mess abit around and (normaly you don't have access to both memory card and internal memory)... If you get a vita2SD you really shouldn't care about the 1GB internal memory for your decision making process if you take a fat or slim model.