Okay, first if you're boiling 3 gallons and coming close to a boil over in a 7.5 gallon pot, I am curious as to how hard the boil is because I can do 7 gallons in a 10 gallon pot and just after the hot break, I do a rolling boil just fine.
But either way, You have more than enough to spend $300 and get both. If you're married to glass carboys you can afford another 6.5 gallon carboy as well as a 10 gallon pot. I like the Concord Kettles found on eBay but you'd want to drill those out yourself because his holes are a bit higher than necessary. If you don't want to bother with that then the Bayou Classic kettles found on Amazon are great. I love my 10 gallon. If you want larger then I think they do have larger. You can easily get what you need for about $300.
If you can only choose one, I choose kettle and maybe a 15 gallon one. I assume you only brew one batch at a time so you can always brew and bottle/keg then brew
etc.