I narrowed it down to the same two. Very happy with the Gateway so far. Better processor, more memory, better graphics chip (note that the geforce 6200 in the m4 is a 6200TE and is underclocked by 30% because Toshiba couldn't figure out how to cool the chip, same with the optional 6600TE) There's NO crapware on the Gateway, and it seems to require far far far fewer utilities at run time than the Toshiba. (omg so tired of 38 toshiba utilities just to make the basic buttons work on the unit)
Downsides are that it's larger, heavier, doesn't have a wacom digitizer, is a brand new design so some people have run into buggy issues.
But also consider what you are buying it for. For mine, I wanted it for 50% office/tablet work, 25% movie watching on planes, 25% video game playing on the road. For that set of usage, the Gatweway is the far better deal. If you don't care about games then the crippled geforce on the Toshiba wont bother you at all, and you might prefer the options that the wacom digitizer has and the square screen on the Toshiba.
For what it's worth I have a friend who has an m4 and he is very happy with the purchase. I think you will do well with either one if you just consider what it is you plan to use it for as the primary guide in evaluating the capabilities.
It's not a bad place to be, you have a choice between two really good tablets. Good luck to you!