I'm 99% sure those stoves are identical with the exception of auto ignite. They both have Room and Stove temp modes with the temp probe wire. I believe they're dimensionally identical, and have the same heat output.
Do you WANT auto ignite?... I don't know, do you? Lol. If you want it because you're intimidated by lighting it manually, or are lazy, don't be. It really is stupid easy to manually light a Harman...as long as you DON'T follow the instructions in the manual. You'll be saving a good chunk of cash up front by going with the P38. During heating season I highly doubt you're gonna be shutting that stove off for anything but cleaning.
BUT........
Auto ignite does have its benefits. At the beginning and ends of the heating season when outside temps tend to be all over the place, it can save you from shooting over your target temps. Once your target temp is reached,th e P38 will continue to run on "idle" mode, meaning the minimum amount of fuel input to reliably maintain a flame. It will not shut itself down. The idle mode is approx 7000 BTU's. Here's an example....its a crisp 50 degrees outside and its gonna drop to 35 at night. Your inside temp is at 70 with the stove on. You go to bed nice and toasty, wake up still nice and toasty and leave for work. The forecast for that day is 70 degrees and you have a decent amount of solar gain and a tight, well insulated house. When you come home, you will likely find your house temp has gone up to 75+-. You now have a few options. You can live with the temp swings and let the stove continue to run, open a window or two and let the stove continue to run in anticipation of another cool night, shut the stove down til later and open some windows, etc. In reality, it's much less troublesome to deal with than it sounds and it's a common "problem" with heating a house with a space heater. From a savings standpoint, are you saving money on pellets by shutting it down on those days? Of course. The trade off is you'll be lighting the stove a bit more frequently during those times of the year. With auto ignite those temp swings are gone, but that comes with a relatively hefty price. Lets say you take the lazy route and just let the stove run."Wasting" $450 in pellets (the price diff between the 38 and 43) leaving your stove on in idle mode is gonna take a loooooooong time. Likely many years worth of burning. In that time with an auto ignite stove cycling on and off, you're gonna have an igniter failure which is $80 DIY, more if you need the dealer to do it for you.
That's my $.03 on the issue. Both stoves have their pros and cons BUT considering saving money is the #1 reason for choosing a pellet stove, spending $450 on that option isn't the best choice IMO. Don't get me wrong, there have been several times I've said "crap, I gotta light the stove"...but with a propane torch or starter gel, 45 seconds later it's lit. Hardly an inconvenience IMO. I would absolutely NOT hesitate to buy a stove that didn't have auto ignite.