I'm guessing your main consideration is $$$. If you haven't flown for a while then you'll probably take, maybe half a dozen hours or so (assumption) to get back in the groove. Once you get to nav stage the conversion is very quick. When I "upsized" for navs it consisted of 0.9 circuits (and that's because the new plane was very different to land).
So personally i'd look at the difference in hourly rates between the two a/c and multiply that by the number of hours you think you'll take to get from here to nav stage (be realistic in your best guess). That's the savings in using a C152 instead of a C172. If the savings is greater than an hour or so in a C172 then there is your answer. If the savings isn't much and you'll blow it in 1 hr in a C172 then start on the C172.
Without checking the rates for the 2 types i'd guess the first option is cheaper. Either way you can pocket half the difference and send the other half by money order to Bill the economist (that's Bill Lumbergh not Bill Evans), this pilot's gotta eat.
And if that just confused the hell out of you PM me
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P.S. your flying school should tell you how many hrs they spend on type conversion but like I said mine was 0.9hrs.