If you go with a vertical whip you will have 20dB of polarization loss working local SSB/CW stations. If you go with a horizontal antenna you will have 20dB polarization loss working repeaters.
I have had a 2m SSB rig mobile and just used a whip. During Es band openings polarization gets changed in the ionosphere and you can still do quite well. For locals *if* they could hear me most base stations could switch to vertical antennas.
If you can only do one or the other decide which mode is most important to you and choose your antenna accordingly. Since 2m FM is the most utilized mode from a car I chose to stick with a vertical antenna, albeit a big one