Welcome to the fold! You'll be happy with your Brompton; it's a friendly, capable little bike that can go anywhere. My Brompton has already put down many more miles than any bike I've ever owned; granted most of those it's been a passenger like me...

I got the T bag for my bike, P6 type Brompton, and would heartily recommend it. The geometry of the Brompton front luggage block only allows for so much room, and the T bag uses all of that room. Normally packed, its capacity isn't much greater than a C bag; the difference is you can overstuff it, and it has the back pocket and water bottle pouch. I love the water bottle pouch and use it all the time; Dutch design, it just exactly fits a 750ml bottle of wine while keeping it clear of the handlebars. Convenient for grocery runs!

I have found that the T bag, properly packed, can serve as carryon luggage for a lengthy trip. Clip it to the bicycle and wheel it to the gate, check the bike like a stroller and walk on the with T bag. It's edge cases like this where the extra capacity of the T bag can come in handy, as well as grocery shopping. Which is fun on the Brompton, by the way: I generally put the bike in a shopping cart and push it around with the groceries. Basically I can't imagine the T bag being too much bag for anyone, and unless you like/need the more classical messenger bag look of the C bag, I'd go with the T. Keep in mind with the C that it still has a rigid metal skeleton and won't carry like a messenger bag at all unless you remove it; it's this skeleton that attaches to the bike, and repeatedly removing it and putting it back into the bag is doable but cumbersome.