You probably don't need to worry too much about the difference in the learning curve. The two phones will both look exactly the. Same (software wise) to you. The only difference is that with the rooted phone you would be able to tweak more things in the future. However, that also means that you could screw it up.
I would personally be more worried about the intelligence of the person who rooted the droid that I was considering. Did he use the root privilege responsibly? Specifically, with the droid 2, did he overclock the heck out of it and do physical damage to the processor?
Almost any software problem can be fixed by a sbf (I stress the "almost" here). I am of the opinion that you should not have root privileges unless you know how to use them. If you were to purchase the unrooted phone and decide that you wanted to root it, you would be able to read a guide here on droid forums and root you phone all within an hour. That would also give you the background knowledge that you need to understand what the rooting is really all about.
In conclusion, because of the possibility that the owner of the rooted phone may have caused internal, phisical damage (I say the just because he had the ability through the root) and because you could root the other phone with ease, I would recommend that you purchase the unrooted phone. Once you know how to use it the way it is and want to branch out a little, then read o guide on rooting and root it yourself. You will find that the root access is much more useful to you that way
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