Hello Scottbral,
The C150 is the better choice for STOL performance in that class of aircraft. The C152 suffers in STOL performance because of the factory propeller choice (though some are changed by now), and the 30 degree flap limit. Now there's good wisdom in the reality that having flaps which are so effective in drag that you get get into runways which you cannot get out of is not such a great idea, but with good judgement, the greater flap travel is still nice to have. For me, every landing, no matter what the wind, is full flap. An 18Kt direct crosswind is manageable, and I've landed with winds 38 gusting to 43Kts. Every takeoff is 10 degrees flap. It is only occasional zero flap for the practice of a failed flap motor.
The choice of STOL kit brand is really not a big factor. I have a liking for the Horton kits, but the Sportsman and Bush kits are quite good as well. The STOL kits come with new wing tips, because of the change in airfoil shape at the tip. It is my experience that these have very little affect, other that they look cool. At one point I lost one (suspect bird strike, but it was night so I don't know for sure). I did not notice it missing until the walk around the next day. There was no change in handling with only one. There are also extreme droop tips. They are not compatible with STOL kits, and in my opinion, provide no apparent benefit. They do block quite a bit of view under the wing! With the installation of a STOL kit, you want to be certain that the propeller is a 48" pitch. the 50" or 52" will give disappointing climb (and thus STOL) performance.
A few comments about flying STOL kitted Cessnas; Aside from higher G flying, there will be very little difference in the handling and performance with less than full power. With full power, you can easily get yourself into a phase of flight where a sudden engine failure is going to be a real problem - you will suddenly stall before you get the nose down to start a glide (you'll already be flying well below glide speed). The aircraft will be very controllable throughout, but you will loose altitude rapidly because of the high drag of the angle of attack. Similarly, it is possible to glide at much slower airspeeds quite safely, except that if the glide is to a landing (which most are), there will be no inertia left with which to flare. The aircraft will change pitch as expected, but promptly stall and hit the ground at the same rate of descent. If you were planning an off airport crash, this might be a good idea, but it otherwise results in embarrasingly hard landings! The C150 will be placarded agaist spins wit the installation of a STOL kit. This is because it can no longer meet the requirement to perform a six turn spin - it comes out on its own after a turn or so. The spin changes to a spiral dive, and thus the speed builds up rapidly, and the recovery is different. The STOL kit does not cost you any airspeed in cruise. The STOL kit will dramatically reduce the stall speed in high G turns. The possiblity of reduced engine cooling at lower airspeeds and igh power is ever present, but the engine cools very well. If you are going to monitor a CHT, put the probe on the aft passenger side cylinder.
The C150 with the 150HP Lycoming conversion is not a worthwhile aircraft for STOL work. It is much heavier, and if the prop is optimised for cruise, it will be very disapointing for STOL work. The 150HP C150 I used to fly had 14 pounds of lead in the tail and this made the low speed handling very mushy in pitch - the last thing you want in a STOL plane!
There are also vortex generator kits. The only thing which is negative with these, is the possible damage resulting from needing to clean the wings of snow or ice, or their interaction with wing covers.
Oh, leave the wheel fairings at home. They seem to have little speed benefit, and a weight detrement - The 10 pounds of mud which each one will carry, that you don't know is there!
If you've got a C150 L or M (and perhaps K), it will have a "key start clutch", unless the starter motor has been changed. These are fantastic units until they start to go wrong, then they are horrible. At THE FIRST SIGN of slipping during starting, do not start the engine with the starter again until the clutch is cleaned by a suitably skilled shop. Special tools and skills are required for reassembly, so not a lot of shops can do this work. Failure to heed this advice (because after slipping, the engine did start afterall, and you went happily flying, and forgot all about it), WILL result in a ruined clutch, and the very real possiblity of circulating very hard bits of broken bearings through the engine's very expensive close fitting surfaces. More detail on this is available if required.
I hope that helps,
Pilot DAR