Should I get a Red Eared Slider?

I really want a pet, preferably a reptile, and the only herp that my apartment will let me have is a turtle. I looked around and I really like the idea of an aquatic turtle like a red eared slider. I've owned a leopard gecko before, so I have a 20 gal tank and a UV lamp. I looked it up and the internets say I'll need a 50+ gallon tank, but I'm only living in this apartment for 2 years, tops. Will the 20 gallons suffice for two years? If not, is there another turtle I can get that could survive well in 20 gallons?