Have a look at the reviews. Image quality is almost identical for realistic uses, so the main difference will be in how you use the camera - the 60D will have more advanced features and customisation, while the D3100 might be easier to use. I don't have experience of the kind of entry-level cameras the D3100 is, the class didn't really exist when I bought a DLSR and I went in straight at the 60D level, which was no problem for me, but I'm equally sure the D3100 would not be all that limiting either for most intents and purposes.
As ever, try them out and go with which you prefer to handle.
Will you regret it in the future? Yes in both cases! Technology moves on and you will nearly always find your camera is 'worth' less and newer better stuff is available down the line.. ignore that feeling - the pictures you take won't suddenly get worse now that a newer camera is out.
Not in the slightest, unless you have some old nikon lenses lying around or want to pick some up second hand. There's more than enough modern glass available however.Is the lack of a focus motor a big issue?