3G is theoretically much slower than 4G. But once again, it really depends on what you are trying to do as to whether that matters to you. If you are on a wireless network then 3G/4G doesn't matter at that point. If you are browsing NSC or reading emails, 3G is still fine. I'm assuming you...
The answer to this really depends on your usage, and whether by 'The iPhone 6 will then be mine' you mean you have one now and will soon own it outright, or that you currently don't have one but desire one as part of your next deal?
If you've got the handset already then absolutely the best bit...