Looking at buying either one of these to hold MISB for a while.  Thoughts on which is the better potential for increase over MSRP after retirement?  

I like them both, I bet they will both be winners. I think the Tower Bridge has wider appeal. It's had a longer availability, I think they'll both retire around the same time. If I had to choose, I'd go bridge. If you don't have to choose, get both.

I'd second grabbing both if it's not an issue of capital. However, if you can only grab one, consider holding time. I would expect, as I think others here do as well, that the Tower Bridge is in a different 'class' of building set, up there with Eiffle, Taj, Statue of Liberty. It may take longer, but I think the Tower Bridge will ultimately plateau at a higher price than ToO, but might take longer to reach that plateau than ToO might take to top out. All that said, ToO is by far the more interesting build. Lots of repetition on the TB, only a bit of repitition on ToO, with lots of neat 'advanced' techniques. My approach to this has been developing and I've discovered I personally like to invest in sets which I want to own/want to display. Both these sets fit that bill, however I've only got the ToO out in a cabinet. 

All this from a novice investor, so take with a couple large grains of salt!  Just my 2 cents on these sets, betting there's lots of other takes on it.

Get the Tower Bridge.

Get the Tower Bridge.

I hope he is not still waiting to make his decision.