It all depends on what you want to do with the unit. If you are using it for standard GPS navigation on major highways, then you might not need the trails. But if, like me, you're using this for geocaching, then you'll probably want the trail feature from the start, just to give you an idea of the terrain you will be searching in.
Be warned: The default mapset that ships with the 450T do not do "turn by turn" navigation, like a Garmin Nuvi does. For that you have to buy yet another map (The North American mapset) and then load in the ones you want.
I see that CostCo online is now selling "lifetime" Garmin map updates, for $135, good for the life of your unit.
This requires more research, as I still don't think you get the topo maps with that. You can also install topo maps from USGS for free, with greater detail, although you may have to run them through a converter program, like GSAK.
Confusing, isn't it? I spent more than a month before I bought my 450T, but I really love it! For paperless geocaching, it really can't be beat. And you can load maps, .gpx and .loc files etc. either into the unit, or the micro card slot, which really makes it easy to update. Be warned, however, that the largest card you can install is "only" 4gb. The unit does not support 8 and 16 gb micro cards yet.
Good luck!
- mvsopen