I had a girlfriend who owned the later 2.8i. This one came with the 5 speed box and was unimpressive. Piss poor bottom end torque combined with tall gearing made the car feel lethargic. Looked great though.
The old V6 Essex lump may have been agricultural but at least it had some low end heft.
They have very similar acceleration; the slightly quicker 2.8i also has a higher top speed which are both down to its ability to rev. If you remove the 2.8i rev limiter it's easy to over rev one. It doesn't have the power to use the fifth gear for a higher speed though. The 3.0l is mechanically limited by it's camshaft, valve train and inlet manifold, anything over 5000rpm requires time, ear plugs and a complete lack of mechanical sympathy. You'll also be resetting the valve clearances all the time.
Both cars use the same 3.02 diff ratio, and the four speed 2.8i used the same gearbox as a 3.0l. Fitting the 3.44 diff from a 2.0l car improves the drivability of the 2.8, and the fifth gear means you keep most of the cruising ability. The same diff in a 3.0l ruins the whole car; 1/4 mile time, in gear acceleration and fuel economy all get worse. The four speed boxes are unbreakable, the T9 five-speed is barely up to a hard driven Pinto, V6s kill them in no time.
Another reason the 2.8 'feels' slower is due to K-jet's slow throttle response. I've written before it feels as if the throttle cable is made from knicker elastic.
Both engines are big, heavy, thirsty and lacking in durability. That last one is mostly down to the nylon timing gear and weak oil pump drive.
A 2.8i is a better car all round; it has suspension that works, a better interior(it's a mix of the Ghia and S specs) and is built into the later, better built bodyshell. That's probably a moot point these days, as any Capri that isn't a five figure car will need all the usual welding; sills, arches, floors, front spring hangers, rear roof posts, windscreen corners, wings, front and rear valences, A-pillars and inner wings. Some of the five figure cars will need much of that work done again, but properly.
I like Capris a lot, but a mildly tweaked 2.0l is a much better car both to own and drive.