Upload & Sell: On
Seems you understand your two EF-S lenses will not be usable on the 6D, so we'll move on to your first issue, crop vs. FF...how do I know it's worth it to upgrade...spend all that money?
Short answer, you need to bring the 6D's images home and work with the files personally, you must view them, post process and print these images and compare them with your 60D's output; results. This will help you decide if the upgrade in IQ is worth moving from crop to full frame.
1st, make sure your current post processing and printing software supports the 6D.
2nd, To most cost effectively review the 6D's IQ potential, features and cost justification, do one of the two options below...shoot both RAW and Fine Jpeg, at as many ISOs as you can...bring your own SD cards, one for each body, keep the images separate.
- rent the 6D and test shoot it, even for one day if you can afford it; read the manual before you do this, you can download it from Canon's website.
- or, a less costly, but potentially less effective option, ask your local camera shop if you can test shoot your 60D kit along side the 6D. Most shops will have a demo body and lenses that will match your current kit and they usually will let you take a few indoor test images;ask if you can do the same outside the shop in daylight as well...if the shop is in a closed mall this is probably not going to be possible, but ask anyway.
Once you've got some test shots, get home and study, post process and print the images, see 1st hand how the images look and feel; fit into your work flow, how well your equipment handles to new files, storage requirements.
I've been shooting for 4 decades and have found it very helpful to find a local photo shop and explore, become a regular customer, even if it's just buying accessories they will help you. I still shop, explore, just look around, in the same store where I bought my 1st 35mm film camera; that was back in 1978
Good luck, most of your questions can be discovered personally, with the 6D in your hands, images for you to work/print with...pixel peep, post process and print the crap out of both the 60D and 6D test images, Make your best effort to do this before you spend your hard earned monies on something you may or may not need.
PM me if you'd like, a 6D will be in my kit around April, I'm looking forward FF greatly...but I have a few years of experience with the original FF bodies/lenses, namely film, to help me decide; I've also shot and worked with the FF 5Dc and 5D2 bodies. Give a serious look at the various 6D treads here on FM, check out the images posted, send a PM or two to the members that are using the 6D and ask them if it's FF images are worth the expense.
Happy New Year