I don't care too much about the weight of the bike, I'll use an old 26 x 1 & 3/8 and put them inbetween the tube and tyre. I mostly ride to school and work and then down canal towpaths or similar surfaces at the weekend so I shan't worry too much about rolling resistance and all that. I have two sets of wheels for the diamondback anyway, one with a skinny tyre on ,not sure what size, and the other with a 2.25, skinny for school, knobblies for the weekend.
what sort of "abuse" do you give your cranks/bike? there are heavier duty downhill cranksets available, if it's the square taper that is causing issues then why not look at changing to a different fitment like Octalink or Isis?
tbh your crank issue sounds like it may well be down to user error.
and what's wrong with your Raleigh?
On the Diamondback Topanga I rounded off the taper in the pedal, this was my fault, on the left crank. On the nameless black MTB I used to take it to the bmx track with my mates and I bent the right crank out to about 30 degrees from straight, so the left crank was donated to the Diamondback, which is checked every friday after school. The Raleigh Marauder, I was coming home one day and I stood up to get up a hill and the one piece crank snapped on the right side and I went flying forwards catching my bits on the stem, the wound was too fresh to talk about previously. ON my old triumph and raleigh superbe they have cotter pin cranks, is that right? They look sturdy might look into them.