For all police, fire, and EMS in Mercer County you would presently only need an analog conventional scanner, with the following exceptions:
Trenton FD, you would need an analog trunktracker to get nearly all of their channels.
Ewing PD and Trenton PD can not be monitored with any type of scanner.
Both Ewing and Mercer County have all-digital trunked systems, however all current users are encrypted so no scanner can receive them. It is unclear if/when there will be digital clear (unencrypted) comms on the systems.
Getting a digital scanner now would expand current listening options outside of Mercer County (including Burlington, Ocean, and Philadelphia), and presumably within Mercer in the future. Don't expect the Pro-197 to go on sale for $300 again any time soon though; a sale price of $400 would be a little more likely.
The Pro-163 is custom made by GRE for RadioShack. It is a clone of the GRE-branded PSR-400 with really only cosmetic differences. ScannerMaster currently lists the PSR-400 for $170 (regular price $200), so I highly recommend it over the Pro-163 for cost reasons. In addition GRE-branded scanners generally come with as many or even more accessories than RadioShack-branded scanners.
Jim