I've tried both and done exchanges myself a few times. I always end up going with the regular size.
The Plus is a amazing phone, and very fun to use, but once you're out and about, day to day, the size starts to become cumbersome for me. Battery life is amazing but for my usage the regular 6 has gotten me through the day. Between the 6 and 6S, and even the 5S, I've never once ran out of juice but the end of the day.
I also don't use the camera a lot, so OIS isn't needed for me. I've summed it up for people that decide between the 6S and 6S Plus:
If you just want a Phone and have a tablet or iPad, get the 6S... You can still consume media, but it's obviously going to be a huge advantage on the Plus. If you want a compact "phone" for calls and text, this is a good option.
If you're seeking an all in one device, media consumption, or in need of very long battery life or the best camera experience, get the Plus.