Well, here's my two cents... And that's probably exactly what it's worth... As for the P-51? Nah, save your money.. They are beautiful planes, but they fly pretty hot... Need to keep speed up... When your used to a trainer, it's very easy to "forget" that not all planes have those same characteristics... You cut the throttle on that P-51 and try to bring her in for a nice slow landing and it'll repay you with a nice "THWACK" as it jams a wing into the ground and does a couple of cartwheels...

I have and have flown a lot of planes out there... Do I think you need to go to a mid-wing, or mid-wing trainer like an easy-sport or something like that? Nah... But you do need to limit your selection until you get used to how different planes feel and react.. Low-wings DO have very different flight characteristics... As do semi or fully semetrical airfoils...
So.. On to my next two cents (woo hoo! that makes 4 cents so far!!!) If I were to suggest a "next" plane for you, it would be one of two...
The GP Ultra Sport 40+ or the Hanger 9 Twist.... Here's the reason... The US40 has a unique airfoil desing... The chord of the wing is much thicker at the fuselage than it is at the tip... The wing is joined at the middle at an angle so that the TOP of the wing is perfectly flat, but because of the taper, that makes the BOTTOM of the wing have diehedral.. Basically what this does for you is this... You have a VERY capable and agile pattern type plane that you will NOT get bored with... It will fly FAST with a higher pitch prop, it will do rolls, tumbles, spins, and loops like crazy, and will be a BLAST to fly... BUT.... First, when built right, they fly VERY stable and smooth... It's almost like it's on rails... Because of the diehedral built into the bottom of the wing, all you need to do is cut the throttle, maybe dial in just a degree or two of flapperon, and she settles right in on a perfect glidescope... It doesn't want to snap and it lands as smooth as can be.....
The Twist is another alternative... It's actually more of a 3D or fun fly plane than a pattern plane, but it also has some great flight characteristics... If you pull the throttle back, in a gentle wind you can literally just about set the thing down vertically...

It's very stable in slow flight and is no problem at all to land... At the same time, you throw the throttle to it, and it's a whole different beast... Vertical climbs are a cinch, rolls and loops happen NOW, and again it's something you'll probably never "outgrow"... The ONLY complaint I have about the twist is that it's designed to be either gas or electric... Because of that, there isn't a lot of wood in it... A hit that other planes would come out of with no damage can sometimes leave the twist needing a little glue and balsa..
I'm sure there are plenty of other planes out there that exhibit these same characteristics, slow stable flight combined with incrediable agility with more speed... But I mentioned these because out of everything I have flown, these are the BEST...
One other comment about the twist... It's a $99 airplane in ARF form! It doesn't get much better than that..