I have stayed in both the tower and the garden wing multiple times.
I don't find the walk time to be noticeably different. When you stay in the tower, you have to wait for the elevator. To me - that is close enough to the time it takes to walk to the garden wing. Now - some of the garden wing rooms are a bit farther away. But - again - to me - it's not a consideration. One of my kids feels as though the difference is almost unbearable. I feel that if I walk 60 miles in a week, that short distance is negligible. If you are going to be running to the Contempo Cafe or the grocery store gift shop (I can't remember the name of it) a lot - that maybe makes the walk a small consideration. But I still say it is a small one. Plus - you can send one person on those runs if you have to.
As far as the cost goes - 3 of the 4 of us feel it is worth it to stay in the tower. On the tower side of the argument it's my daughter who hates the extra walking, me who likes the view and my other daughter who likes everything about the tower. My husband just sees the dollars floating out of the sliding door and would rather stay in the garden wing. To compromise - we stay once in the garden wing for every two or three tower stays. We call it our white trash vacation when we stay in the garden wing.
We only stay facing the park. I don't feel that facing a lake is a big deal. I can do that near my home or on any number of other vacations. Plus - you can get a lake view from the garden wing.
So - not really a definite answer - in part because they are all great places to stay. If I had small kids, I would go tower so that I could easily watch the fireworks when they want to sleep. I feel that staying in the tower facing the park is the ultimate in Disney resort stays. If you can swing it financially, it is worth doing at least once.
I will add that you will be dealing with elevators and the stroller in the garden wing unless you get first floor.