Either way I sweat buckets during the warmer months of commuting (Clyde here), but I still choose racks to save my back having to shoulder any unnecessary weight (it has plenty of weight to carry as it is). Topeak Explorer on all my commuters and the Topeak MTX trunk bag EXP with fold-out panniers. I will be trying some Ibera panniers soon, just to see if they'll be a decent fallback option.