I've heard that Boss product used with both kinds of amps. Here's my take on it.
The GT-8 (and similar products like the POD XT, etc) were actually designed to go to a PA system directly. In other words, they work best when the output is not colored by a tone stack or any amp distortion. So, between the two choices you present, the solid state amp would probably give you the most faithful reproduction of the sound.
That all said, products with digital signal processing are always going to have some small amount of digital artifacts left in the signal, even after going through the final D/A (digital to analog) conversion. In other words, there is going to be a small amount of square wave still left in the signal.
Tubes have a tendency to smooth that out without significantly affecting the high end. Tubes will also add some amount of compression to the signal. All of this contributes to what guitarists refer to as the "feel" of an amp. That is, the interaction between the guitarist, the guitar, and the amp.
This was the idea behind the Line 6 Spider Valve, as it uses a Class AB tube power amp after all of the digital processing. I've played it, and it does "feel" more like one of my old Fender or Marshall amps.
So, which is better? It's really up to you. For the truest fidelity and low maintainence (no tubes to wear out), go with solid state. If you're used to playing a tube amp, stick with that.
Greetings from Austin, Tx
Ken