Which one do you prefer? Typhlosion because of its typing which I prefer over Charizard's.
Which is more useful overall? Hmmm... Charizard has the better movepool but Typhlosion has the better typing in my opinion. Charizard has a x4 weakness to rock types and Stealth Rocks just ruin its day. But in the other hand, it's immune to Spikes. I'm still going with Typhlosion though. Mostly because of typing.
Which one would win in a battle? It really depends on who is faster. If Charizard goes first and uses Earthquake, it might take down Typhlosion. If Typhlosion manages to survive or goes first and uses Rock Slide, it's likely that Charizard will be taken down.
Overall my vote goes to Typhlosion. This was one of the hardest choices I've took in Pokemon.