Performance vs Gaming

Which is better: The XPS 8700 or the Alienware X51? They have similar specs so I think the XPS 8700 would be better since it has a bigger case which means better cooling. (At default configs)

Alienware X51 Gaming Desktop | Dell with 7200 rpm HDD with GTX 645 vs XPS 8700 Performance Desktop | Dell with 7200 rpm HDD and 8gb ram but with GTX 635

Neither is going to be an amazing gaming device.. the GTX 645 would be the better card.. but its still not a very good one, even gaming at 1080p youll be limited to low to medium graphics settings on most games.

In reality, I will go with a XPS 8700 with 750 Ti. I can't compare this with a i7 version of the X51 since it only allows R9 or 700 series cards. The XPS 8700 will already be over powered for Minecraft which would be the most heavy game that I will play. (I don't like FPS games like BF4)

Also, the reason I chose such underkill computers are because I could not find good comparable configs for X51 and XPS 8700. I just want to compare the XPS 8700 configs to the X51 configs.

Why go with a gaming PC if you're only going to be playing minecraft, you could get away a celeron and basic onboard/gpu if that's all you want to play. But if you want to future proof you would be better off either building a full stock machine yourself or if it's out of your league get someone to do it for you. You would get a much better spec with a great reduce in cost over the 2 machines you posted

Because I will use a lot of mods though which does need a fast computer to get a decent fps.

I mean, if you look at a YouTuber that plays mostly mimecraft, many of them have very overpowered computers. For example, AntVenom has a Intel 3960X with 2 Nvidia GeForce GTX 680s.