Both can be used. Mij and jou are stressed, me and je are unstressed. Ze houdt van mij. Ze houdt van me. So when do you have to use one or the other? In simple sentences you can use whichever you want, in sentences where it's stressed you cannot use the unstressed version. I guess it's easiest to just give an example: Ze houdt van mij, niet van jou! (She loves me, not you!) Wrong: Ze houdt van me, niet van je. Hope this helps.