Why not combine?
Training till solo on the 150, basic instrument and night flying on the C172 and maybe a training flight to a towered airport ( if you training airport isn’t).
This was you’ll have 7-10 hrs on the C172 under your belt for rental after you get your Private certificate.

You may not safe $2k by the way.
C150 will cruise at 90mph flat out down hill with a tailwind if you’re lucky and the C172 will cruise at 120mph.
If the practice areas for the maneuvers are 20 miles away then it will take more time aka money in the C150 trucking back and forth from the airport to your practice areas.