I have both. I have had MW1 since about 6 months after it came out and still play it every day. MW2, I got it the day it came out, played it for a month, haven't since.
I played through the single player and thought it was good, but only because I was looking forward to that more than multiplayer. However I played through again to youtube it and show a friend (he only has a wii so missed out on it) and thought how crap it is. After the first couple of missions, it is the same old same old, they take what was great about MW1 and explode it massively so it doesn't impress any more and for a "realistic" game, it is horrible unrealistic.
Online, if you are on PC, don't get it because of the lack of dedi servers. The matchmaking system is alright for consoles, but for PC it is a hell of a fail. You will get a laggy host almost every other game, you can't have any kind of clan gaming/competative gaming because of no dedi servers, you can't enforce rules or have a niche server like you could in CoD4, you can't kick hackers, of which there are so many because the VAC system doesn't work there and then, it works way in the future when a new, improved hack that bypasses VAC has been released.
Aside from no dedi servers, it still doesn't work online. They took the whole it is whoever controls the weapon best/knows the map/has the best reactions away completely. you play CoD4, the majority of people are running about, in MW2 it is all campers because the health is much lower and guns do more damage, so they just have to keep their sights over the entrance and get the kill, so anyone at all can get a bjillion kills with 0 skill. For alot of people, that would just mean throw a nade into the room or sneak up on them or snipe them, but when you have people in that are crap at the game anyway and are running about, you get stupid amounts of people having a little cry or rage quitting, which wouldn't normally be a problem, you ould tell them to shut up and play or leave, if they didn't you would kick them, but again, you can't kick anyone, so you are stuck with them.
You have to pay ridiculous amounts for DLC,, the game initially costs way too much, if you are on PC you can't sell it on because it gets bound to your steam account, unless you make another steam account just for it then sell that, but then you are breaking the EULA so that isn't allowed, so if it turns out you don't like it, you are stuck with a game you don't like, that you payed stupid amounts for, that you can't mod to make it any better and that you can't, unless you are stupid or too rich, get new content to make it fun again
=EDIT=
oh, I should point out on CoD4 there are modded servers with the guns and maps of MW2, so you can get the game from there effectively for free, and you get the majority of the DLC from MW2 too because they just remade CoD4 maps anyway