Training Plane.
 

I start flight training this weekend. The flight school Has a C150 for 100 wet and a C172 135 wet.

I wanted to train in the C150 because it's cheaper but my CFI suggested the C172 Because it safer and faster.

I would be saving around 2k using the C150.

is there really a big difference ?

For PPL, no the 150 is the plane thousands have started on. Save the money for serious instrument work.

How much do you and your CFI weigh together? The 150 has a useful load of about 620#. With 38 gallons of gas at 6.4# per gallon and 6 qts of oil at 8# per gallon that leaves roughly 365 pounds. If either your CFI or you are over 200# the Cessna 150 will definitely be no ball of fire in the climb out, especially on a warm day (not that a 172 is either). Of course you don’t HAVE to top off the tanks.

Everything else being equal, hire the lightest CFI you can find. ;)

What altitude do you live at. In Denver the C152 is only good for light weight students and CFI’s. Sea level? I say save the money, and go with the 152, assuming you’re not overweight.

Sounds like the CFI wants YOU to pay for HIS on the job comfort.

Unless as others have said density altitude is an issue or you're very heavy. If the CFI is too heavy, get a lighter one.

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.