Well, I've taken both with me on quite a few trips and truthfully, with the Tenkara weighing so little, there isn't much of a penalty with regards to weight (4oz.). Even though I enjoy getting a spinner out to deeper water, using Tenkara has been amazing.
With Alpine Lakes, so many of the larger 'cruisers' are so close to shore, particularly later and earlier in the day. On our last trip, I actually caught just as many and of good size with my 12' Iwana than I did with my spin outfit. I used to think that Tankara was going to be used for mostly rivers/streams and the spin rod would be for the lakes but, I have changed that paradigm.
I am considering just taking the Iwana on the next trip into the lakes and I am confident I'll have no regrets. That said, I just love fishing too much to not want to take both rods on trips. I'm now having to ask myself that if my main goal is to catch as many fish as possible (never really feel this way), than spin has the most flexibility and best chance of accomplishing my goal. If, on the other hand, I want to just catch some fish and simplify the experience...I go with tenkara. By sight fishing near shore for large cruisers...I've done extremely well and had more fun doing it.