I'm thinking your 700 will have more power then the 550 efi.
"The twin is smother running then the single cylinder" this is a quote I've
read but don't remember who said it or if it's truly true.
You say efi would be nice, do you have any problems with your carbed
700?
Again you say your 700 runs & rides great just got the bug for a new one.
Do you have any mods done to your 700? If not not, maybe some
upgrades to the 700 can make it seem like a "New" machine depending on
what may be available for that year & model. Thinking off the top of my
head some larger tires, clutch kit, new exhaust...you get the picture.
But if you got the bug & the cash, then go for it