Thanks all! That was very helpful. Car stuff gives me a mental block, for some reason.
After much asking around, I found a well-recommended local garage (which is actually a big relief, as I've been frustrated with our previous place). The guy running it took the time to explain what made each tire option different from another, and found me a pretty good deal, without being condescending, so it's a win-win in my book. Of course, we'll see how the actual tires feel/sound/last!