I liked both. But I think the interface of Witcher 1 is an order of magnitude better than Witcher 2. Navigating the inventory, journal, alchemy, etc... is cumbersome in Witcher 2. It's very console-ized (I guess since it looks as if it is being ported to the XB360).
Combat wise, I think Witcher 1 was a little better. Witcher 2 combat is fine, but in Witcher 1, once you really learned to use the timing system well, you felt powerful like Geralt is suppose to be. In Witcher 2 you feel very normal just like any other soldier on the battlefield. And for all his agility like you see in cut scenes and finish moves throughout the game he sure does pause a lot in actual combat. You can't take a step then cast a spell like arrd without a very noticeable and annoying delay. I could understand it when he did a evasive roll, but from just taking one step seems very unlike the character. I also think the potions use was much better in the first witcher. I didn't spend even 1 point in alchemy tree because potions must be taken before a fight. The first one did potions correctly IMHO. You take them anytime during combat, and have to watch your toxicity levels or suffer the consequences.
Story wise, hands down the second is much better. I haven't read any of the novels, but I did enjoy the story of the second one better. The game does feel shorter in Witcher 2 but the quality and tightness of the story is very apparent. There is very little grind, and I did try to do every quest on my first play through.
The quest system is very well done I can't off hand remember the first Witcher quest system, but I did like the no hand holding quest system of Witcher 2. I loved that at least one of the quests required you to talk to townsfolk so that you could get info that wasn't written down you had to listen to it and remember it. If you failed to remember it correctly or didn't talk to them, you'd have to fight rather than talk your way to your goal if you couldn't answer back correctly.
I thought the way they handled sex in Witcher 2 was vastly superior to Witcher 1. It felt more in place and fitting in to the story, rather than just a "how may can I conquer" mini-game.
Overall it's a toss up IMHO. Story - Witcher 2, Interface/GUI - Witcher 1, Gameplay - depends on your play preferences. I think people who like traditional RPGs will prefer witcher 2, while people who like fast paced action games will like witcher 1 better. I do like fast twitch action and fighting games so I liked the combat system of witcher 1 better. Witcher 2 combat felt much more reserved.