Your post makes it sound as though you are very new to riding, but you mentioned mountain biking. If you are good with taking drinks from your water bottles when riding then go with bottles. If you're not really comfortable riding your road bike yet, I might consider getting a bit more familiar with the bike before you join a group - can you change a tube if you need to? (assuming that someone else has a pump or CO2)

My seat bag has the same stuff whenever I'm riding, only comes off for a race - spare tube, CO2, tire levers, multitool, spare chain link, patch kit etc.

I'd take the Garmin. Way more useful data to analyse afterwards, and you don't want to be looking at your bike computer on your first group ride, you want to be paying attention to those around you and riding predictably.

Good luck and have fun.