Why not a bit of each???
I agree with the time is time is time....scenario. If you are "TIME building" take a very slow aeroplane and go a long way with it. You'll get time.
Having said that, I would hazard a guess that you will LIKLEY take you commercial course on a Piper...(bet some will scream at that...I did say "likely")...and, personally, I would consider learning the relevant machine at standard PPL flying school prices rather than at commercial school prices - whilst also eating into your course flight time....you can use that time to learn what you need to learn to pass the skill test.
I would be tempted to go get some hours in the 152 - but use the time seriously. If you just want the magic 150 hours then why not get an aerobat and screw around the sky for a few weeks burning holes in your wallet and filling in the log book.
If, however, you wish to pass a commercial skill test then I would contemplate going on long and involved cross country flights...keep altitude to within 50 feet of your own nominated level...keep speed within five knots of your own nominated speed and don't ever let the aircraft go more than five degrees of your calculated heading unless you say, out loud - in advance, that you wish to change the heading to a new figure.
I'd also take the aircraft into the circuit and practice practice practice at nailing the trimming for any nominated speed...preferably the speed nominated by your future training school for the particular phase of flight that you are currently in.
If you cannot maintain the 50', the 5° or the 5 knots...the hours will still go into the logbook towards the 150 you want (or whatever it is) but you should consider the flight a personal fail...and do not book the commercial skill test (yet).
BTW...what is the current cost of doing a commercial ride nowadays? A complex aircraft for two to three hours, maybe plus positioning, maybe plus landing fees, examiner fees, parking fees, day off work....what does it cost to do the skill test?
All above is just my personal opinion...but that is what you asked for (I think).
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