They're both very different pens aside from the fact that they are both German piston-filled pens.
Pelikan likes to use bonded parts and the piston mechanism is a snap-in part. Oftentimes when the piston is pushed out, it may not be able to snap in securely after that. The grip section is bonded on the barrel, possibly chemically. In some pens the piston mechanism does come loose, but it is not common.
The Lamy comes apart without any real snap-in parts. It's a very efficient design that mostly screws together. Even the cap comes apart this way. It's one of the few modern piston filled pens that doesn't have a screw-in, snap-in, or friction-fit casing that holds the piston mechanism parts.
In any case, you should not have to really service the piston. Both pens use urethane pistons that don't necessarily go bad. Lubrication can be done without ever taking the piston mechanisms out of the pen.
Another important difference is that you can easily get replacement nibs for the Pelikan. For the Lamy you have to send the pen to Lamy to get the nib replaced. I also don't recommend unscrewing the grip section of the 2000 unless you absolutely have to. In some of the earlier models that had the grip sections with a full metal liner with a black Makrolon accent, the metal grip section threads could chew up the threads on the mouth of the barrel. I saw at least two earlier pens where the owners had unscrewed the grip sections often enough that they completely stripped the threads off the barrel.
Makrolon is also a tougher material than the plastic Pelikan uses. I wouldn't recommend dropping either pen, but I have had to send a few Pelikan pens out for warranty repair because of cracking since someone dropped their pen or their pen fell. I do like the lightness of the plastic construction of the Pelikan. The Lamy, ever since the full metal grip section with the Makrolon accent has a bit more weight, and the balance is more nose-heavy than the original with the Makrolon grip and metal accent. I like the way both versions balance, but they are different.
Finally, the Lamy has solid stainless steel trim without plating. You do not have to worry about the plating getting damaged or wearing off. Overall fit and finish on the Lamy 2000 is more similar to that on the M400 and better than the M200.
If you are looking for something modern-looking and durable, I would get the Lamy 2000. If you want something that looks more traditional in look and feel that's lighter, I would get the Pelikan M2xx. They are both excellent pens with different pros and cons. Get what you like and enjoy.