If you want to use your tube amp for tone / sound (not tube clean), you either need to blend it to use the Amp Sim (with the SAG parameter) or bypass the Amp sim completely, so the Axe-Fx is essentially a multieffects box then. At least that's recommended in the manual.
As long as the power amp (SS or tube) has headroom and provides clean power, and you don't want tube power amp based tones color, then either is fine IMO. In this case, with clean power, you could use the Amp block sim. (this is what I did pre-FRFRs) Other than hi-fi tube power vs clean SS, not sure if the extra cost for tube power, where your tone/sound/color is from the Axe-Fx, why you'd spend the extra cash on tube power?
Tube free here, and luvin' it, frankly.