I do not unfortunately own a Lamy 2000 just yet but based on what I've seen so far I can offer this input:

The Lamy 2000 has several distinct advantages. The first is that it is made in Germany. Their standards are quite high and so their workmanship is excellent. The materials are of high quality and there are no cheap components. The nib is hooded and made of 14k gold which, although not essential, is nice to have. Apparently it is lovely to write with as well.

The Sheaffer is a good pen, so I don't want this critique to colour one's opinion too negatively. I like it as a daily driver and the writing experience is very good. It works well for me. The Sheaffer is made in China so the standards are lower. Although the body is made of metal and quite sturdy, the grip, feed and large parts of the converter are all of a lower grade plastic. You can still see the mould lines on the entire grip piece and it seems cheap. The nib is steel and of good quality. The converter seems to be good. It fits well and is snug with no leaks or anything. It is transluscent so you can see the ink inside, but you do need to remove it from the body as their is no ink window. This pen is far heavier than the Lamy 2000 at 45 grams overall vs. the Lamy at 25 grams and more than half of that weight is in the cap. This does not bother me personally, but I've heard that as a complaint.