Tuk-Tuk, on Samosir, is very, very touristy. It's popular with Indonesian tourists as well as foreign tourists, and so has various irritations catering to them (like speedboats on the lake). Tuk-Tuk is the only place that I have been in Sumatra which I didn't like because it was too touristy.
If you really want to go to Samosir, there are other places to stay as well, away from Tuk-Tuk. Apparently there are some nice traditional villages around as well. But they will take more time and effort to get to.
Maninjau is not touristy at all! It was touristy some 20-30 years ago. The tourist numbers have dried up significantly since then. It sees tourists, and there are a few guesthouses around to cater to them. But there's little tourist development around the lake, absolutely no tourist hassle, and not even many tourists, at least when I was there. Very different from Toba, and in particular, Tuk-Tuk.