The 152 does have a few more horsepower than the 150, and often a somewhat more coarse pitch propeller, so a little faster cruise can be expected. In a 150, 2200 RPM is on the slow side for cruise power. 2400-2500 will get you better results, particularly if you are leaning. The mixture distribution is more even at 2450+-, than at engine speeds faster or slower.
70kts does sound a little slow though. It could be that the prop on the 150 is pitched to a very low blade angle, for better climb. There are also wing adjustment cams, whose position can slightly affect cruise speed. I presuppose that the engine maintence is adequate. The O-200 is a fine engine if well cared for, but low cylinder compression can be a problem, and it will reduce power output. The O-200 is very unhappy running on 100LL, and this was another reason for the change to the O-235, which was very happy on leaded fuel. The 80/87 of the "old" days ran very well in O-200's. Though 80/87 was permitted to contain lead, it rarely did, so lead fouling was not a problem in the O-200, until 80/87 was no longer available. When 80/87 became difficult to obtain, and 100/130 became the "norm", the O-200's suffered terribly.
The flaps are different between the types, not in the selector (the few 1977 150's had the same "preselect" flap control as the 152's), but in the angle. All 150's have 40 degrees of flaps available, all 152's have only 30 degrees. That last 10 degrees is very noticable for short field work. The problem is that the aircraft is easily able to land into a runway too short to takeoff later. It also makes full flap overshoots a little challenging, though contrary to what you will regularly read here, a 150 will safely climb away with full flaps. I think that Cessna changed their thinking to no longer optimize short field performance, and more tend toward simpler trainer type performance.
Another difference between the types is that most if not all 152's are 24 volt electrical system. The O-200 engine was not available with a 24 volt starter and alternator.
You can play around with getting on the step, but the affect is much less noticable on the 150/152, than on other types. It's not a bad Idea, but not a big help either.
I hope this helps, enjoy the 150
Pilot DAR