I have a Rolleiflex "T" with a 3.5 Tessar. I usually shoot at f8-f11, and this lens is painfully sharp. When I compare it to my 2.8/80 Biometar, 2.8/80 Xenotar, or 3.5/105 Colour Heliar, I really can't see much difference. All lenses are in excellent condition, so lens condition is not a factor. The "T" has one of the latest formulations of the Tessar design. As mentioned in the above posts, some of the earlier tessars were not as sharp as the later designs.
I have a shot that I took with the Rolleiflex of an old outhouse where the paint is flaking off the wood. People often remark that the 8X10 looks so sharp that it seems you can pick the paint chips off the print with your fingernail.
If price is a consideration for you, the prices for Rolleiflexes with the Planar are significantly higher than models with the Tessar. So long as you don't shoot wide open, a Tessar in good condition will give you outstanding results.