Hi I have ridden both.
I have also raced a 784R, and ridden the F4 SPR and a 749R on the track a number of times.
Apart from the obvious difference that the 848 is a newer design and that it is a twin vs a 4, this would be my take.
The SPR 750 F4 is special bike, similar in feel to a 748R but with a longer and higher top end, so on the track it is a step up.
The 848 is lighter and has a more compact and modern riding position (the F4 is more flat and racier.).
The F4 has higher spec components and a lot more adjustability than the 848. Also the F4 is very well designed with bodywork fit, and finish etc superior.You can change to track bodywork very easily, while the 848 takes abit of spanner work.
The F4 is a 4 and a high revving one so you need to work it, take your 848 and add about 2000 revs to everything and you get the picture. It is smooth and absolutely screams at high revs, sound totally opposite to the drone of the Duke.
If you can swing it and have both it would be a perfect garage mate. They handle and feel simiar but the engines are differnet in the way they go about their business.The SPR came at the wrong time (just when everyone was going to 1000's), but it is special for sure. If you don't want a 1000 you won't be dissapointed.
Goodluck.