IMHO, such a question is a bit meaningless for the Pi's. The main purpose of a SBC market is to get a general purpose computer for a low price. The compute, IO, etc. performance is limited only by what you're willing to pay.

E.g. the 4Gb Jeston Nano has a lot of compute power compared to a 1Gb RPi, but the price of $99 vs ~$30 means it's not really an "alternative" as much as it's a product intended for a different purpose.

You can even get full blown x64 PCs in roughly the same size. They'll blow away any of the RPi line, but at a price.