I will say THIS IS IT! Frank is very humble when recommending equipment, he usually recommends the less expensive when there is more than one option, he's a frugal guy. At Magnapan they answer the question of what do you pay for as you go up the ladder of their products with the answer "more resolution". The 550 is a hybrid, designed to extract the most from each. I've had mine for 7 weeks now, so when I say THIS IS IT, it is for me. BTW, at shows, Frank displays the 550 in the prime system.
If the difference in price is not a problem, THIS IS IT.
Seriously, I can't speak for Frank, and this is your personal decision, mine was the 550 and I bought an extra set of tubes.
Marty