In my limited experience with helm seats, they always seem like they are in the way, and aren't positioned correctly for someone to sit at them while running the boat. This is just my generic opinion of helm seats on all boats, just not Parkers.
In my first boat (17' Bristol Skiff) I chose not to have any seats at all behind the console (there wasn't room for a leaning post). I found it very rare that I wanted to sit down while running the boat, and there were plenty of other spaces to sit if I was at anchor or something.
My current Parker has the leaning post / livewell combo, along with 4 rocket launchers. I think this is an optimum setup for my boat. When on long runs, I can lean against the leaning post and take some of the pressure off my legs. When I really want to sit while driving, I can sit on the leaning post. The livewell is a good size and doesn't take up any additional footprint. The aft edge of the leaning post has a grab rail. When seas are rough and I am drifting while fishing, I can easily lean up against the grab rail without my feet hitting the livewell.
A friend of mine with a Jones Bros had a Birdsall leaning post seat installed. From what he told me, most of the aftermarket leaning posts had a very large footprint, providing space for a large cooler underneath. The Birdsall model took up much less space, but still had room for a cooler. It can also be ordered with a backrest. It has storage under the seat cushion, but I don't think this particular model had rocket launchers. I think this would be a good choice if you were also going to get a T-Top, so you could have the rocket launchers on it instead.
I have had no experience with a T-top either, which is why I ask the questions. I am a caster on rivers and bays, basically. I troll a little, I drift a little. I don't have the sun considerations in New England like those who are in sunny places (Florida).
I do not like my console seat. I don't sit on it except when drifting, the rest of the time I have nothing to take the pressure off my legs. I see the leaning post as the answer for this issue.
My previous boat was a 17 Dusky Open Fisherman, and it came from the factory with a leaning post that would hold a 72 qt cooler underneath (which I only used as dry storage).
My experience was similar to Toms. Great to lean against when running, and a good place to sit when drifting with my feet on the washboard. Mine had a nice removable pad on it (removed in the photo above) and room for 5 rods.
Didn't have a t-top, and never felt I was missing anything.