The Uniden and GRE radios each have strengths and weaknesses. As for mobile vs handheld, that's something you need to answer yourself, based on what you want to do while scanning. If you want to carry it around places where you won't have either AC power or a car, you want a handheld. Handhelds work just fine in cars and homes as a quasi-"base"/"mobile". There used to be a better argument for a base/mobile when they were more robust and had different hardware inside, but these days, the 996XT is just a 396XT inside a larger case, and same with the PSR600 vs the PSR500, more or less. The one thing to concern yourself with is legalities - I don't think NS/NB have any issues with mobile scanning (I think, if I recall correctly, that MB is the only province with issues with that).
The Aliant TMRS (which I presume is what you would be listening to) is a Smartzone system. This doesn't matter in 99% of cases as far as scanners go - to scanners, it's just a "Motorola Type II" system - but if you plan to roam about the province, in my opinion, the GRE does a better job of handling switching to different sites in a multi-site trunking system. The GRE actually measures signal strength for all the control channels it can hear (I'm pretty sure of this) and uses the strongest one, while Uniden's design simply scans each site you have unlocked and tunes it in if the control channel can be picked up. So you could theoretically monitor a weak, distant trunk site with the Uniden, and get comms cutting in and out, whereas on the GRE PSR, you're much more likely to get a stronger signal.
One thing I noticed when I was taking a road trip thru Newfoundland a year or two back, was that the GRE appears to be a little poorer at picking up VHF signals than the Uniden, on stock handheld antenna. Aliant is an 800MHz system, so if you're listening to that, that's not so much of a problem.
The GRE does take a bit of mental gymnastics to get used to (the object-oriented memory storage) if you are used to the Uniden way of doing things (with systems having groups and groups having talkgroups/frequencies). In the GRE, every "object" - trunk system, talkgroup/conventional frequency, etc - can be linked to one of 20+ scan lists. So scan list 1 could be for Fire and contain both trunk talkgroups and conventional frequencies. On the Uniden, each system's groups are independent of one another, and each system can only contain trunk data OR conventional data, not both. So you could have one system with group 1 being fire, and that group having conventional frequencies, and another system (this one trunked), also with group 1 being fire, and that group having trunk talkgroups. You could switch either of those on or off at either the group or system level. On the GRE, if you have scan list 1 on, it's on across the board - trunk talkgroups, conventional freqs, search ranges, etc.
Also, Uniden has geolocation features (able to turn on/off systems by GPS coordinates if you plug in a GPS to the scanner), which I don't think GRE is capable of.
In short, the decision is yours to make - you need to lay out what exactly you want to do with the scanner, and evaluate each based on its strengths and weaknesses in each category.
Hope this helps!