I've just re-read that sentecne and it is confusing.
Which sentence is that?
I want to make a robot with 6x legs. But I want to develop a leg for the robot, an axon would be required for the full robot, but for just something to quickly prototype a leg I'd use something a bit simpler and cheaper.
Does that make sense?
I guess you may think, that it's easier to learn/use a simple controller, but actually, you'd just have more to learn, as you cannot apply all you've learned to the "full version", unless you use the very same controller, so starting simple might be starting complicated.
You shouldn't fear neither speed nor "features". If you don't intend to use eg. SPI initially (or later), just don't, the controller doesn't force you to learn all the peripherals that you don't need, so just pretend they're not there (until you need them).
If you only need to control say 3 servos initially, you can
do that with eg. a PICAXE, a BASIC Stamp or whatever, but then you'll have to start from scratch when you move to a larger controller.
With eg. the 83 lines of a ChipKit32 you can just ignore the 80 pins and control the same 3 servos with the same amount of work as on any other controller - and it will then be simple to extend that code to 18 servos when you're done with testing the first leg.
But hey, it's your money and your time spend learning one or two systems, so it's your decision