I used to think it mattered to me, but I've found that it doesn't, and that I (apparently...??) don't notice any difference.
I look around the web and find whatever is currently the least expensive.
I DO care about the diameter, and I DO care about the flux...Otherwise (I suppose...??) I just naturally adjust heat/timing.
I've also known people to buy a spool from Hobby Lobby (in the stained glass area, like Mark mentioned) and it works just fine. They used to have a 40% off one item coupon, but I don't know if they do that anymore.
Jordan King 2341 with Holton Monster Eb Bell
King/Conn Eb Frankentuba
Pan AmeriConn BBb Helicon
Yamaha YBB-103
"No one else is placed exactly as we are in our opportune human orbits."
I can even use that thick no lead solder from the hardware store in an emergency, but I neither like no lead nor thick. Notice that our Berlin friend observed that two different alloys we're just fine, which sort of goes along with my post.
As far as that so-called plumbing solder is concerned, I got an entire sousaphone ready to put back together and refinish one time and realized that I had no solder. I put the whole thing together with that and to the best of my knowledge it's done fine to this day, and it's been quite a few years. I didn't have much fun sticking it together with that stuff, but it worked. I also use that stuff for repairing tiny cracks on crappy old school instruments whereby the band director has made it very clear that they just wanted to play for cheap. Actually, I've also used that plumbing solder to trace along spider cracks on first branches of sousaphones - sousaphones that probably should be thrown in the trash.
I'm not going to tell you about the crappy torches that I use. It's really more of a matter of experience and having a touch - I think - than having the very finest torches.