The 5100 got a bad rap. It wasnt around long til the 5105 came along to replace it. There is TONS of reading on the 5100 on other sites about its dilema, but here is it as I understand it:
When dolmar came out with the 5100, it was a screaming fast saw. It was set pretty lean from the factory to get that screaming fast speed AND to meet the EPA emissions. It was "assumed" that dealers would richen the saw up a bit to prolong the life. But many dealers did not. That in combination with a poor air filter desigh, and a lot of them toasted themselves. Thats why dolmar came out with the 5105, that was tuned a little more conservitive, AND new air intake design. THat is MY PERSONAL understanding of it.
Now, that said, the 5100 is an excellent saw, IF you richen it up a bit AND IF it isnt already toasted.
IF this is a good dealer, ask them about it. They "should" be honest about the 5100's negative rap. And at least ask them if they did a comp test and/or pulled the muffler to see if there is any scoring and if they tuned it "right".
As to the prices.....are they new or used?? Makes a big difference when it comes to saws. But new, the 5100 could be had for ~$400, and the 420 around $300.
The 420 OTOH, is one heck of a sweet and smooth trimming saw.