Which would be generally more useful? Small handle ball end hex drivers or larger 'plus' size? Looking for a 1.5/2/2.5/3/4mm set
Depends what you need to do. Beyond the Wiha set linked above, I also have a few Wera hex drivers (0.05, 1/16, 1.5 in micro and the big 2.0, 2.5 and 3.0).
I really like the handle of the Wera drivers, big and micro. The micro drivers are similar to the Wiha Picofinish.
The big difference between the micro and the big drivers is how much torque you can apply. The Wiha or Wera micro are very nice to screw/unscrew small metal screws in metal holes or shallow plastic holes (laptop, phones...). They are made so that you can press on the top of the handle while quickly rotate the driver with the fingers. The variable diameter along the handle allows you to tune for speed or (some) torque. So they work well for speedy screwing/unscrewing at low torque and a safe lock due to the downward pressure on the top.
The big drivers are meant to be used with the whole wrist and the tops don't rotate. Basically for much higher torque applications like screws into deep plastic.
In my case, I use the Wiha set for most electronics need (appliances, laptops, phones...).
I use the Wera big drivers (and wish the 1.5 hex was available in big driver) for RC Cars. These tend to have long screws deep into untapped plastic holes and require substantial torque. The micro drivers are basically unusable in that application.
Note that I find that the handle and tips of the Wera drivers to be superior to the Wiha micro, but the Wiha have much more variety available (especially if you're need SAE sizes) and still work very well.
As far as ball hex vs. straight, I like the straight hex for most applications as they are more stable, but the ball hex are more versatile.
For the big Wera drivers in particular, the straight hex have a slightly pointy "Hex-Plus" shape that I find works extremely well in high-torque applications to prevent slipping and rounding of the screw heads. It is an acquired taste (often you have to reverse the driver pretty hard to disengage it from the screw). It doesn't look like the ball heads have it.
http://www-us.wera.de/hex-plus0.html