I'm a person who prefers the Phenom II's over current bulldozer architecture. regardless on the outcome, you will have a later option to upgrade to FX Piledriver chips in the future if you ever need to.(assuming this is on an AM3+ board of course)
Which Phenom II X4 is it? The Phenom II X4 965 is better at gaming than the FX 4100 and it also beats it in most other applications as well. Also keep this in mind. You shouldn't think of the FX 4100 as a quad core CPU but rather as a dual core with hyperthreading. I'd probably get the computer that has the FX 4100 if only because it has an AM3+ motherboard so you can upgrade later if you want to do that. Really there isn't a huge difference between the mid range Phenom II X4's and the FX 4100. If you can do it I'd seriously consider a different PC which contains an intel 2nd or 3rd i3-i5-i7 generation CPU. Really even the G series Pentiums are pretty good.