Undergravel filters do have their uses. They're very gentle, so good for fry tanks, and they have a very large surface for colonisation by the bacteria.
But, tanks with undergravels need stripping down on occasions (probably yearly) to clean out the gunk that accumulates under the plates, and plants don't do very well with them, as they don't like the flow past their roots.
If you do decide to use one, it would be better to run a reverse flow system with a powerhead, rather than the standard air driven version, but I'd probably stick with internals if I were you.