I just went from 2 trucks down to one so my perspective might be a little different. Originally had an FJC that was built and then purchased a LX450 that would be my primary trail running/long distance truck. Having two built up vehicles seemed like a waste to me as I wanted to continue building them both but didn't have the funds to do that. Sold the FJC to get the fiance a vehicle but I didnt like DDing the LX as I felt I was wasting it. Bought a new F-150 which gets surprisingly good mileage (20-21 on the highway) and have been using it as my exploring (not trails) vehicle and DD. Listed the LX for sale because I wasn't using it much anymore and didn't want to maintain two rigs in addition to my fiances. Now that it's gone I really miss having two vehicles and the luxuries that provides (can fix one and drive the other, etc).
That being said, we own an 06 100 that does not do very well on gas so chosing that as a commuter doesn't seem like the best idea. Weight will be your biggest issue as you build the 100 or a tacoma as mileage will shrink drastically. FJC got 20 mpg on the highway and when I was done it got 15. LX was a bit of a different story but averaged 13-14 on the highway.
Ultimately, do what makes you happy and enjoy!
2013 Toyota Tacoma DC
2006 Toyota Land Cruiser
Searching for a clean 1996-1997 Land Cruiser