The more teeth, the larger the cog.
The larger the cog, the lower the gear.
That, if you are climbing steep hills then 25T could well be a bit ambitious (depending on the steep) equally 32T is comparatively big for a largest rear cog so should get you up anything (my Synapse has 28T, the flat bar Sirrus is 30T). It's likely to also mean a wider spread of cogs, which probably won't bother you. Means you'll notice a bigger difference in 'pedalling difficulty' when swapping between them.
Sounds like you want an 11T or 12T at the bottom for what you want to do (a bunch of people say 11T is pointless, I quite like it on the Sirrus) that'll allow you to crack along at a nice pace on the flat.
Incidentally, considering what you're looking to do on the bike, I'd add a water bottle and carrier to the 'accessories' list. Doesn't have to be technical, but you'll want liquid if you're going out for any period of time.