As always, some good points made by all sides. From my years of building and racing these things, I have found that it really comes down to personal preference. While alot of today's .21's will have more than enough power and low end torque to move a truggy out in a timely fashion and as Max stated, will get the power to the ground efficiently and on alot of surfaces will have better forward acceleration due to more efficient power delivery. As far as a .28 not having more rpms is not always true. I have raced with megasaxon personally down in PA and he has personally watched my Nova Plus28-7T with my standard race port circle the track in a very timely fashion and with the proper tire selection, it was laying the power down very efficiently for a modded .28. But with more torque comes more clutch and gearing setup. Most stock gearing in truggies are up in the 15+:1 ratios where the buggies hang around the mid 11's somewhere. Add in the added torque but with increased tire diameter, do the math and select the proper ratio. As Max also stated, the bigger engines are harder on clutch shoes, bearings etc. so a little more maintenace is in order when running the .28's. Some of my fastest engines have been built out of .28's and i also have done alot of side by side testing in the same vehicles with same clutching and gearing and from my experiences with my own engines, my .28's always prevailed everywhere as far as power output and rpm. On some tracks, a .28 will be a better suited engine ansd some tracks, a nice .21 is the best choice. some of my team drivers have run both sized engines in their vehicles and have reported back with similar opinions. There are always 2 ways to skin a cat so this is in no way me telling anyone what is best, just stating what I have found with my engines, in my environment. I do alot of traveling and I always run a .28 in my truggy and never have a problem with too much power. It just takes a little different approach to setup and operation of the bigger engine. I run .9's in my truck with stock gearing or 1 up on the bell depending on track size and conditions. Sometimes I get more wheel spin out of the corners than someone running a .21 but there are areas on the track that a .21 just won't always cut it that i make up the ground needed to stay competative. Last but not least, it depends on driving style as well. I drive like Ryan Lutz with my truggy so the .28's go perfect with my style and I have been to tracks were my .28 engine will clear triples and quads that a .21 falls short on also making up some lost ground so as I mentioned, will depend on what you think your driving style will support and what the track will allow. I'm sure this converstion will continue on course and it is always good to discuss things with each other. Just remember these are only opinions so no need to get all fired up over it. the way i look at it, if what you do works for you, then great!! If you want to share with others, thats cool but don't think that what you do is gospel and everyone else is wrong for what they use and how they use it.
For the guy who originally asked the question, if you feel a .21 is the way you want to go, my recommendations would be a Nova P5, Plus 4, Jammin JPX .21 or something along those lines. If you want a good running .28, the Nova Plus28 series is good for the money, a Picco P3-28 is decent as well. The Plus28-3 is a great engine for the money but will need some more blowdown timing to get the upper range performance out of it. The Plus28-5 is decent all around but also will gain performance with some port work and the Plus28-7 is great as is and is even nastier race ported. The Picco P3-28/jammin JPX .28 is wicked with a race port and it actually requires a nice port job to get the run time and tunability out of it. The stock engine can be finiky and run times are not very good due to the low end needing more fuel than normal to compensate for original engine design. Run times on all of my .28's have ranged from 9-11:30 minutes after mods depending on pipe, tune, climate etc.