STATIONARY and ROAM settings only adjust which control channel the scanner should use and not how well the scanner decodes. If simulcast and non-simulcast channels are both entered a PSR-500 scanner will tend to lock onto the non-simulcast channels over time as they are more consistent in signal strength while simulcast channels vary in signal strength at any given time (due to multipath distortion).
Uniden scanners are less prone to interference and decode a little better on weaker signals than GRE scanners so tend to work slightly better. On simulcast systems, you basically are trying to get a good signal from one tower while avoiding reception of the other towers on the same control channel. So, a directional antenna works or sometimes just laying the scanner down flat or setting it next to a metal object that blocks signals from other directions. Simulcast systems are tough for any handheld except the more professional units.
I use both PSR-500/600's (phase one only scanners) and a Uniden Trunk Tracker IV {update a BCD396XT} (phase one only scanner) on our simulcast system in Weld, Colorado. Some days are worse than others for reception and I almost have to use the Uniden Trunk Tracker. I have also used the more expensive Uniden BCD436HP and SDS100 scanners however found no real improvement over the simple Trunk Tracker IV on our system when testing side by side and driving around the county.