I think your plan to train in the garage is perfect. Even at that age its important to start them on the road to a routine when it comes to potty training. I also had winter pups (2) at two separate times, not my idea but we managed. I kept the potty area inside and worked on that but when we had a break in the weather we always took them outside rain, snow or whatever, we were right there with them through it all encouraging them every step of the way. So if it were me I would do both. I would suggest that you set up both potty areas as close to the door that will be used as you go in and out for potty time. That a key step in training your pup.
I've trained lots of puppies in my days and have taken the time to add my routine to my website if you are interested in reading it.
www.dawnsyorkies.com/training-tips