[3v3rnoob]

While it's true that you want some variation on one robot playing defense and another playing offense, consider what you want each to do.

Offensive robot:
Pick balls up off the ground
-how else are you going to be able to score?
Score balls
-what did you think you were going to do with the balls once you picked them up?
Drive
-because the balls are not going magically move into your picking-up device
Aim
-well, that's not essential, but it's a little more reliable than driving to the point where you're confident the shot will go through

Defensive robot
Interfere with the opposing alliance's offense
-whether that's physically blocking shots or bumping them while they are not on the key, you need some way of doing this
Send balls to your alliance's scoring side of the field
-if you can't do this, what's the point of blocking?
-to do so you will need to pick balls up and throw them
Drive
-Yeah...

Anyway, on paper, these look like similar robots. The former is likely going to be more sophisticated, but there is no reason that a robot capable of playing offense will be incapable of playing defense.

You can do both.