Like many things, it's something of a tradeoff - the GRE/RS scanners are known for having problems in high RF density areas, but their P25 decoding is reportedly better than the Unidens. On the other hand, the Unidens tend to do a bit better in resisting overloading issues in high RF areas. It should be stressed, though, that given high enough levels, any handheld - from any of the 3 - is likely to have issues.
Now a word about antennas - handhelds aren't generally built to take the higher levels of signals that say, a discone mounted outdoors, would provide. The little antenna is very much a compromise in most cases (and that's being polite), so if you're in a high RF area, and decide to put the handheld on a larger outdoor antenna, you should plan to put some additional filtering on it, or there's a chance the handheld might be overloaded.
What are the Uniden digital handhelds? That would be the BCD396XT and its older brother the BCD396T. You can read something about each by clicking on the model number found in...(note that all links are blue)
Uniden Scanners - The RadioReference Wiki