The age'd old decision.
At the end of the day it is your choice. But I can say the difference between a 4.8 and a 5.85 is terms of shear size is a lot on it's own. Add to that the fact that with a 5.85 extended cruises are not out of the question, where with a 4.8 you might consider it but I wouldn't recommend it.
Then again, trailering etc. A 4.8 could be towed by anything. Where a 5.85 as long as you have a reasonable slip 'anything' loosely used as a towing veichle will be 'ok'.
Fuel costs between the two, I would guess are marginal. But performance wise there we be a big difference.
Add to this the fact that it is a buyers market out there and there are some real bargins to be had. Then in hindesight, a new boat is a new boat. So you have a dealers warranty on the hull and machinery as well as electronics.
But I'd guess you can get a lot more than a 5yr old 5.85 for the same price as a new 4.8. That would be around £10k yes?
And i'm no expert, just my views and thoughts