Hard decision, but i am rooting for wifi only.

We have seen what the chaos theory did to Jolla, the more time and complication you bring into a product, the more will go wrong.
All of us would be happy if this thing takes of in any form whatsoever, so better go for the low hanging fruit and play safe in this attempt.
No one will be unable to have mobile internet, so the "problem" this 3g onboard convenience solves, bears the dangers already mentioned:

1) risc of incompatibility with vanilla jolla updates
2) uncalculatable impact on battery life.
3) more variants for you to produce and market
4) many more propabilitys for murphy to kick in.

please play safe, play successfull.
If you still think 3g is doable safely, i trust you and would buy.

EDIT:
@cy8aer & MartinK i am using the tablet for navigation on hiking trips because the display is great for showing the route to family. Works great with modrana and picks up position very reliable. I even have mounted a 7" car holder and occasionally use the tablet with here we go. No problem that would need improvement imho.