Bullfrog is in a class above Viking from pretty much every standpoint. Viking has a shady prorated warranty and is generally considered a mediocre brand at best. I know they've added some fancier, "higher end" spas to their lineup and they looked a lot nicer but I'm not sold.
While you're looking at Bullfrog's X series, it's still every bit the quality of their A and R series spas, just no jetpaks and a shorter warranty. Bullfrog's warranty is full parts and labor for the entire duration. Prorated warranties are b.s. in my opinion and deceptive.