I've not flown the Grob (although have several similar aeroplanes, including the Bulldog and T67), but have plenty of hours in C152 which I think is an excellent learning platform. My thoughts nonetheless..
- In the grand scheme of things, adding a VP prop doesn't really make it much more complex
- Odds are that they're probably both perfectly good training aeroplanes
- If you enjoy the aeroplane more, you'll probably enjoy the learning journey more, and thus do better at it
- On the other hand, the Grob is doubtless more expensive.
On the whole, I'd say you're choosing between two good options there - go with your gut feeling, and you're unlikely to be disappointed.
G