I ran my 21SE for 6 years with the carbed 150. IMO, thats plenty of horsepower, even with my t-top and a full crew. I went thru plenty of seas and came back in thru many rough inlet runs in that boat and never once felt worried about safety or lack of power to punch over the waves.
With regards to dryness of the ride, sure, you might get wet if you push it, or get the wind blowing back spray on you if really windy, but if you don't mind taking the time, youll get there and back.
As far as comparing to the 21 SeaCraft, my good friend has one, and I have fished on it alot. It does not have near the amount of room as the 21SE. That Parker bow area is second to none. I got my new 23 without the high bow rails, and that makes it so much nicer for fishing up there, anchoring, docking, etc.
Sure, the SeaCraft is going to ride thru the chop better and faster with its deadrise, but there have been many times when I have felt uneasy on that boat while drift fishing in some pretty nasty swells. It does whip and rock alot more that the Parker. Heck, the first time I jumped on the boat I almost fell over as it really pitched, and I'm so used to jumping on my parker and it not even moving. I have no experience with MayCrafts
I would second sea1skiff's opinion about looking at the 23 parker for not alot more money. I'm into my second year on the 23, and it really is a big, big difference in the ride over the 21.