I recommend that you look at the stickies thread for direct comparison between consoles.
I will say this though that the HP3500 console is tiny, whereas the HP2551 console is colour and much larger and all round superior.
As for Ultrasonic/Wind cup debate, it all depends on where you reside. I am in southern UK, where overnight temps rarely go much below -5C and so far my WS80 has not dropped a beat and was a pre-production unit so does not even have a heater plate installed so I could not use it if I wanted to! I use it for my prime wind readings and it always is much more accurate than the wind cups sitting 12 inches away I use as a reference.