The -3a is going to be more responsive. Not twitchy, just lighter on the controls, even lighter than the -7. They both are a real joy to fly, but there is something very special about the -3. A solo tail dragger is a real cool feeling, and a challenege ever time you fly. No two landings are ever the same. Don't worry about the first landing, you'll do fine. Take is up and stall it a few times and see where the airspeed numbers are at. Do some slow flight first at 20 over stall, then 15, then 10 to get the feel of the plane. Get to know it, listen to what it is saying to you, talk to it, feel it. Practice a few simulated go arounds at altitude so you know what it is like to dump the flaps. Don't be in a hurry for your first landing, take your time and get to know the plane. When you are "comfortable" land it on grass. Work your way up to the hard stuff. JMHO. Man, I miss that plane!
The first landing I made in a -3 was the best one I ever did, never could duplicate it again, but I tired! 3 point landings were by the numbers, with the tail touching down a split second before the wings stalled. If you were carrying any extra speed she would fly again so you had to be ready for a little excitment, but putting the stick in your gut stopped it. With the extra weight of the 0-320 she should be balanced a tad better than mine, but that depends on your wasteline. Mine had an 0-235 with a well pitched Catto prop. It worked well togeather, although the extra HP you are talking about would be fun!
You fly the -3 until the hanger is close, and don't forget it!
RV-3a N66AZ is for sale in Columbus, NE, if anyone is interested send me an e-mail and I will forward it to the broker there.
Good luck, I hope I helped.