Well, I haven't been playing the Slav for too long, but I wouldn't think that capturing on c4 should be something you do if you "feel compelled" to do it. You should probably know when to do it (i.e. the main line Alapin Slavs) and when not (most other things, like Slav Declined, Schallop/Pin Variation, etc.) If you get that down, it's a solid opening.
I don't play the Semi-Slav at all, but I play QGs as white, and I would definitely say that the openings are very different. The Slav is an opening where you play to minimize white's advantage and achieve a simple game where black doesn't have a lot of weaknesses. The Semi-Slav is often sharper and bolder, but you have a bad bishop at the same time. There are a lot of sharp ways for white to play the Semi-Slav (i.e. early g4 lines, Botvinnik Slavs) and for black as well (i.e. early c5 or e5 pushes, dxc4 followed by ...b5 and queenside play). Meanwhile, even white's most aggressive tries against the Slav (i.e. 5. e4 gambit, 6. Ne5 line) can be blunted by the solidity of black's structure and his logical pieces. It depends on what you want. If you want to play a little more aggressively and enjoy interesting imbalances, the Semi-Slav might be a better choice. If you want to equalize and get a simpler, safer game without too much risk, the Slav might be a better choice.
On the other hand, the flexibility of the Nimzo is the main reason why it's still probably the most common response to the QG complex, and if you really feel neither of these openings is for you, there are so many ways to play in the Nimzo/Bogo/QID that I'm sure one of them will be to your liking.
I can't decide between the two. I really like having a more active light-squared Bishop in the Slav, but I usually feel compelled to exchange on c4, usually after 3.Nf3 Nf6 4.Nc3, so that I can actually develop my Bishop without worrying about Qb3.
But, once we get started in the middlegame, I feel very uncomfortable without a single centre pawn on the 5th Rank.
I have 'The Slav: Move by Move', but I'm considering switching over to the Semi-Slav, which I don't know much theory on.
I do know that I like playing against the Slav better with White pieces. I'm just looking to get your guys' thoughts on the subject--I know they're both playable, and especially at my level, the difference in strength is about negligible.
Then again, maybe I'll just drop the Closed Game and learn some Nimzo or something.