The weight difference between a 15” and 17” isn’t an issue when I’m out and about, it’s entirely a marketing ploy. If it’s heavy your muscles will grow and get over it.
For me it wouldn’t just be the weight, though, but also the sheer size. A 13” Pro can be subtle and fit on a small lecture fold-out table, while a 15” or even a 17” generally tends to become a bit of a distraction. Then again, those really are beautiful machines
Whatever you do, I strongly recommend against anything 15” or larger. 13” is an ideal size for a laptop you intend to actually carry with you through college or university. Sure, if you’re just going to be driving to work each morning and set it up for the day a 15” will do fine. But when it’s in your backpack wherever you go, you’ll appreciate the form factor of a 13”.
That said, I’m currently using a 2010 Macbook Pro 13” that has lasted me through the past four years of university, and I intend to use it at least for another few years. At home, I plug it into a larger monitor (but I don’t have a desktop machine on the side), and at the uni I usually rely on the (ubuntu) desktop machines that are available there when I’m working for longer periods of time.
If you’re getting a Macbook Pro and you’re considering the 2012 model, do add an SSD. It’s an easy and cheap upgrade, and honestly, when have you used the DVD drive of your current machine last? Personally, I’ve taken out the superdrive and mounted it in a cheap SATA enclosure I got from ebay.
I guess it really depends on what you’re going to do with it. The Pro is a more powerful machine, but that does reduce its battery life a bit and makes it a heavier carry. Personally, I’m not sure if I would get the retina model, as I appreciate the storage space of a SSD + HDD combination. Then again, if you’re looking for 8GB of RAM and the latest processor, the retina would be the way to go.
It really depends on what you want to do with it. Does it need to be highly portable? Do you need processing power? Do you need RAM? Do you need storage, or is there a desktop/server/HDD on the side?
For a convenient spec-comparison, check out this tool
Will a Username change affect my sales?
I think that really depends to what extend your branding depends on it. If you have a characteristic avatar, and that is what people know you by, your name might not matter that much. It also depends on how large the change would be, I suppose.
I’m not sure if there are any statistics on the number of ‘return buyers’, for whom it might actually matter, but depending on the market you’re targetting that should also be taken into consideration.