It also depends on what your needs are. If you want a built-in MIDI synth on the card, you might choose the Turtle Beach or SoundBlaster cards. If you are a PC gamer you might do this too, as they support environmental audio and have a game port for a controller.
In modern fast PCs, an on-board MIDI synth is less important, as soft synths can fill in very well if you have a fast CPU and lots of RAM. Of course if you have external synths and/or samplers, an on-board synth chip might have very little appeal.
If you just want to record audio, and you don't want or need the on-board synth chip and the gaming integration, consider something like an M-Audio Audiophile or Echo Mia.
If yopu want to be able to record more than two tracks of audio simultaneously, you will want a multiport interface like the M-Audio Delta series, Echo Layla or any of a dozen or more other choices. More $$, of course...
Finally, just want to point out something in your terminology. You said, "Internal means it just plugs into a sound card slot, right?" "Sound card slot" is a misnomer. Sound cards plug into standard peripheral slots in the computer. Pretty much all modern ones are PCI cards, and connect to a PCI slot. Older cards used to plug into a different slot called an ISA slot, but these are pretty much obsolete and few motherboards (if any) are made with these slots any more. External cards like the Extigy that you mentioned typically plug into the computer via the USB port. These work OK for stereo I/O but USB has limited bandwidth, so they do not support multiple simultaneous audio I/O. This will be different in the near future as USB 2.0 supplants the original USB spec, and as FireWire (another type of high-speed interface port) becomes more common, but with the currently-available devices it's an issue to be aware of.