the 2.8C has quirks with setting the shutter speed and the shutter/aperture lock controls that make it slower to operate than later models.
The 2.8C does not lock aperture and shutter speed, the 2.8D and 2.8E do.
Moreover, the 2.8C has a 10-bladed aperture lacking on every later model (which have 5).
The main downsides of the 2.8C are: the plastic locks of release button and flash cable, which are less durable then the metal ones for later models;
a non-removable viewfinder, which was introduced with the 2.8E2;
strap lugs which are not yet of the scissor type (introduced with the 2.8E).
Otherwise, a fantastically capable camera.
A 2.8E2 would also be really great. But with all of these models, condition is most important.
If you do not want Tessar, presumable because of sharpness, only the Xenotar is a match (if not more) for the Planar.
Yet next to my 2.8C I now use an MPP Microflex with Micronar 77.5mm (Taylor and Hobson) lens for its OOF rendering. Sharpness is very good, if perhaps not beating the Planar/Xenotar, but it is a 5 element 4 group lens unlike most other TLRs which are Tessar copies. Still, I had the 2.8F Planar and sold it, not because it was not good, but because I preferred my 2.8C pictures and got more money for the 2.8F.
My experience with the Rolleis is that people react very positively to the camera on the street. A TLR more than an SLR. I have only received positive comments, in plenitude. And it is so silent! With a good strap, it is easy to handhold with slow speeds as well.
I cannot comment on the 3.5 models. Tests I have seen suggest the 2.8 ones are sharper, but the difference is minimal and depending on condition/sample variation.
Screen brightness is important to consider too. I ended up replacing all original Rollei screens (MX [sold], 2.8C, 2.8F [sold]).
But there is no doubt that the Rolleiflex TLR is a great camera to use on a trip to Europe.