If the pre buy looks the same on both airplanes, similar engine times, and the -6a panel is a glass panel you’re happy with, why you you not go with the -6a? The improvements made in the -7a are mostly in the ease if building due to advancements in machining parts. Possible exception - the -7 has slightly more headroom because of shorter seat pan ribs. I used -7 seat pan ribs in the construction of my recently completed -6. The -7a can also handle a more powerful engine than the -6 because of it’s bigger (and not as attractive) rudder and VS, but it sounds like that isn’t a consideration on your case, since they both have the similar engine. The RV6, -6a, 7, 7a, 9, 9a - all have the same dimension between the canopy rails. They also all use the same canopy. Jeff Rogers has but one canopy mold for all three airplanes, whether slider or tip-up, so the canopy sits on the same fuselage dimensionally. If your goal is to sell it, get the -7a. If your goal is to keep it, buy the one that fits your needs now without “improvement$$”.
That’s my opinion of course. Good luck - you would like them both.