there is a lot of debate going on whether the Fantom dyno is a good dyno or not. (It is said to by really RPM biased and not produce "real-world-like" results)
Also it only tests at 5V instead of 7.5V.
The Robitronics is certainly expensive but it is a really good dyno.
Keep in mind the older types have a 12T limit and even a good 12T can be too much for it. Robitronic can upgrade it for about $100 though.
I have the newer version and it doesn't have the 12T limit. I just dyno'd my new Fantom 6x2
(270watt & 58.000RPM @ 8degrees of timing. Anyone know what I should be getting?)
If you can find a Fantom for $250,- or so, just give it a shot. If it doesn't work for you, you can sell it for the same amount