Big buttons = nice!(providing the sweet spot is spread across the surface, ie. Not limited to one precise spot in the middel of the button)
I'm a bit confused with the question though. Since the mockup seems to indicate that hardware design is already decided upon, there's only minor differences possible such as barometer, software...it becomes confusion for the buyer as to why such minor differences warrant a different model.
My suggestion would be, if you wish to build a diverse portfolio of products, to differentiate more on other factors (hardware) - example a larger, higher-resolution screen. Or, why not, a devise that is built for both bike and running, so with both a solid steering bar connector AND a good wristband.