Man, those are both pretty basic. I guess the X-18H is newer so that's a plus. Other than that, there's not much to say about those. They're both pretty similar, a couple inputs, a trim and pan knob and some faders. If you want to get a 4-track I'd recommend the Tascam 414 or one of the 424's in that series. The cost difference won't be a huge difference and you'll get double tape speed, which makes for better sound. I'd save my money personally, but if you're going for one just make sure it doesn't have anything wrong at all, meaning don't buy if the seller says "I don;t have a way to test it but..."
Does sound quality differ from model to model? For example, does X-15 lack not only features, but also sound quality in comparison of X-18H, X-28H, or Tascam 424? Thanks.
Yes it does. Like Level Anything said, with better models, you'll get double tape speed, which will improve sound quality significantly. There will also be better-grade preamps, etc. in the machines too. I very much second Levels recommendations.
Thanks! By the way, I found that there's no pause button on 424mk3. I assume that I can't let it get in to standby mode before recording to adjust level, etc. Is there any way to pause this device?