Obviously, the 28mm is stiffer than the 25mm (or is that too obvious?) although, from memory, the stiffness is related to the cross sectional area of the bar, so it's not a linear increase.
The 25mm Eibach bar I had on was definitely stiffer than the Neuspeed I have on there now, to the extent where I would get significant understeer and snap oversteer in the right (or wrong, depending on your view) conditions. Plus, the Eibach hit the back box.
The Neuspeed bar is adjustable, mines on the middle setting (25mm ARB on standard +80,000 miles suspension) I've never been able to find out the relationship between, say, the 28mm on softest setting and 25mm on stiffest. But I can tell you I'm more than happy with the 25mm on mine - it's difficult to describe, but it's like it's not there, but you can tell that it is. The tail doesn't wag and there is significantly more grip.
Not sure if the 28mm is "better" or not, but I'm happy with mine, especially as it's a road car.
Edit: Oh, and no extra creaking on mine