For the quality thing...there is the 23.976fps vs. 24fps problem. Most movies are shot at 23.976fps, while the i3's IGP will only output at an even 24fps. What this means is that you will get 1 duplicate frame every 42-ish seconds, which you may or may not notice. Personally, neither myself nor my wife have seen it as a problem. Some people notice it, others don't.
If you had spare parts laying around, I would say to use whatever processor you have. Since you don't have an extra chip, I'm recommending the i3 just because I have always been a "best performance for the price" person. Right now, that's the i3 for $100.
You can't really go wrong either way. Either you have a more powerful chip (i3) that's overkill for strictly HTPC applications other than faster encoding (if you do any), or you have a less powerful chip with discrete graphics card that you won't use for gaming/3D anyway, that will be limited in performance should you decide to repurpose the box down the line. Obviously, it's completely up to you. Personally, I'd just go with whatever the best deal is that you can find that will work for you.
If you had spare parts laying around, I would say to use whatever processor you have. Since you don't have an extra chip, I'm recommending the i3 just because I have always been a "best performance for the price" person. Right now, that's the i3 for $100.
You can't really go wrong either way. Either you have a more powerful chip (i3) that's overkill for strictly HTPC applications other than faster encoding (if you do any), or you have a less powerful chip with discrete graphics card that you won't use for gaming/3D anyway, that will be limited in performance should you decide to repurpose the box down the line. Obviously, it's completely up to you. Personally, I'd just go with whatever the best deal is that you can find that will work for you.