Wow, that's low even for today's standards. If you do get by with 100mb on your iphone, then I think you'll still get by with 100mb even with your apple watch as it only mirrors notifications from your phone. Also, if you might save up on data if you use the mini app on your apple watch instead of the full sized apps on the iphone which might load high res photos and UI data.
EDIT: Assess your usage, you might get talkative with SIRI with your watch now more than ever.