Tricky one, this.
If you lined up four or five recorders of the type we discuss here, and connected them to the same pair of reasonable quality mics, and recorded the same performance from the same position, being careful to end up with recordings that were all equally loud, then you'd probably have to do a fair bit of listening to determine which was the best. However, if you used for or five mic pairs with any one of the recorders, you'd hear much more of a difference. And if you used the internal mics built into the recorders, you might well hear an even bigger distance.
In other words, it's the mics that make the biggest difference. The actual recorders are, to some extent, not so important.
The difference between mics isn't just the frequency response and amount of noise they produce, but also the stereo image differs a lot between different types of mics, and how they are placed.
You need to ask yourself whether, in the context you've described, you really want to be dealing with mics, cables, mic stands etc, or whether having the whole thing in one one piece - using the built in mics of a recorder - might be less distracting (your chief task is to perform!). That also has the advantage that normally you don't have to mess with setting up mic angles and distances and so forth - you're more likely to get something on one occasion that can be compared with another on a level playing field.
If you do go the one piece route, there are some good recorders with bad mics out there, and some indifferent recorders with good mics. Once again, I'd stress the importance of the mic. For instance, the Sony M10 is a fine recorder, but its omnidirectional mics are not good for the purpose you have in mind. The Zoom H2N is less well regarded as a recorder (though personally I'd say most people wouldn't have a problem with it if blind tested against the M10, but it can be regarded as being a reasonable stereo mic system which records (rather than being a good recorder with a mic tacked on as an afterthought. Those are not the only choices of course.
Lastly, I'm a little troubled at the thought of you changing your playing according to what you hear on a recording of this sort. I'm not sure it's going to provide a real impression of things - a recording always falls far short of hearing with your own ears, which as you say are in the wrong place when playing, but musicians have coped with this for hundreds of years before recording came along! Maybe kick the idea around with fellow musicians and get their reactions.
Cheers!