Portal 1's pacing with its puzzles is better than the second game's, as Portal 2 can sometimes feel like it drags on too much getting through one room to the next, just to get to the next part of the story. Whereas in the first game the puzzles were definitely the highlight, in the second game they seem to be more of an annoying obstacle meant to pad out the game rather than a welcome and unique challenge. GlaDOS' subtlety and sarcasm are also much more funny here as one doesn't yet know that she's more murderous than HAL, and it's hard not to laugh when she passes off something threatening so casually as if it were something normal, simple and everyday. In the second game it feels a bit more forced.
The story and setting are better in the second, with a richness about them thanks to the exploration of the testing rooms from the past, with an eerie mood brought about by the obvious signs of decay of the Aperture Laboratories, and not to forget the excellent character development as well of GlaDOS, Cave Johnson and Wheatley. The electronic music is much more prominent in the second one too and I think as such it compliments the futuristic technological mood of the setting well (though Still Alive is a better ending song I will admit, its lyrics are much funnier and threatening than Portal 2's). The multiplayer is very welcome also, the emphasis on actual teamwork right from the start (when playing splitscreen both players must co-operate and press start at the same time or else the game goes nowhere) is what a co-op game should be. Not just single player with a second player but its own game and experience in its own right.
I wouldn't say either is necessarily a better game than the other but I would have to say I enjoy Portal 1 more than Portal 2. The pacing is definitely the affecting clincher in the overall experience for both and the first does things marvelously by being so short and sweet, whereas the second game drags on to the point where I was asking "ANOTHER freaking puzzle?!" way too often in the second half of the game. If the first game was that sweet and sinful slice of cake, the second was a full course meal of dessert which sounds ideal at first, but as it drags on you start to get a stomachache and wonder when it's going to end. Both are great games though and most welcome in my collection, and it's best to play and enjoy them both if you can.