This Easter, we visited Yarra Valley and Mornington Peninsula.
IMO YV was a better day trip, with many quality wineries along a couple of easily navigable roads.

Mornington Peninsula is more spread out due to the topography.
You might prefer YV being a couple of hours (return) closer than MP.
If you do a tour on a minibus, remember they can get unfomortable very quickly.
Smaller, luxury tours might put you in a more comfortable vehicle like a Mercedes or VW mini-mini bus. That' s an option for the Hunter Valley here in NSW, so I think there would be a similar for YV & MP.

I can check some of my tourist info brochures if you'd like more info - just let me know.

A couple of YV wineries have produced international award-winning Shiraz wines, which were my pick. Leave the Cab Sav to SA, IMO.