I have come to the conclusion that I probably suck at this, since I do not get any more sqaddies killed on Classic than on Normal. Which is to be read as "too many already on Normal". However Classic is definitely more fun, since the enemy AI is much better.

I am reloading one or two times on hard and very hard maps because I am trying to learn. I have not played a turn-based game such like this since Laser Squad on my Amiga 500 100.000 years ago.
That said, I only keep one save per battle, the starting one. Because I want this to be a learning experience. I also try to only reload if I make an obvious mistake, not if I have bad luck (if I am in the right position, but my heavy misses a 95% shot then gets killed... I do not reload. If I dash to the wrong place and do an obvious mistake, I reload, because I want to learn the game).

As for the discussion above... I have very limited experience (I am just past my first council briefing, just built my second satellite uplink on my first ever Classic game)...

Personally I find Assault to be the opposite of Snipers (makes sense, huh): Assault is the best class on tight maps and indoors. Sniper is the best class outdoors on open maps. That said, the maps I have the absolutely hardest time with are maps where you have to storm a building. It makes sense realistically, I guess. The enemy have fortified positions with cover everywhere.
I find that I tend to get the best result by taking cover behind whatever is closest to the landing craft, then blow up one wall in the building with a rocket or grenade. Then let them come to me and advance sloowly.