If your mod will allow for multiple bat set ups, I would suggest starting out with the 350's for the reasons you described. You can stack them (again if your mod allows, i.e: Vamo) and get approximately 18 hours, as apposed to approx 17 hours for a single 650. Or you can use a single 350 in shorty mod mode and get 4-6 hours. And before the battery police jump on my numbers here, these are the approximate bat life's I'm experiencing with my Prodigy V3 mech and Vamo V3. Your times and experience will directly relate to how you vape, the resistance of your topper, quality of batteries, and how you take care/charge the batteries. So, IMHO, I think starting with the 350's for now and getting 650's as the vape budget allows will give you great results and be more versatile for you in the long run.