Todd, I am glad you stopped by and happy to hear that you have been happy with the Henderson unit you bought.
You asked what size of machine I seed with. I have both a 300 gallon unit and a 500 gallon unit. Sloshing isn't an issue at all. I am most always hauling them totally full or totally empty. There isn't a bit of sloshing. I have on occasion pulled them half full and don't recall if there was sloshing or not. I could see where it would but if it did it was not bad enough for me to pay any attention to. I only use an F-150 to pull my units.
I can agree that with one 2" jet it would have more power than two 1 1/4" jets. The fewer jets you have the more power you will have but the less even the mixing action. On the small units that would not be all that noticeable and shouldn't be a problem. My comments were based on a quick look at some photos that don't show a lot of detail. My concern was more with the placement of the jets but I can't see what they do with that but based on your comments it sounds like it is fine and that is great.
The only people I have seen use leg tanks before for a hydroseeder were some home built units. I have helped a few people with suggestions who were building their own units and if they asked about using a leg tank I recommended they didn't. A leg tank is much easier to build a frame for but my concern has always been that material would build up in the legs. It seems like a spot where granular fertilizer and mulch could collect, harden and then split off and go into the suction line causing a clog but you seem to be getting along fine so that is the important thing. Evidently it isn't a problem.
I did like their frame design. The wheels would be a nice feature on a smaller unit. My 300 gallon unit weights about 800 pounds and I have 200 feet of 1 1/4" hose on the unit that when full of water, mulch and seed probably add another 200 pounds. I am not sure that handles and castor wheels would be much help loading but for a small unit that could be quite handy. I have loaded 100 gallon units by myself without wheels and it is pretty easy. The back is light so I just set that on the tailgate then picked up the front and shoved it in. It isn't hard without wheels but yes, that is a nice feature.
I believe their price is still the same as what you paid and that is a decent price. Briggs is a good engine and yes, with the smaller leg tanks their are isn't anything they can really do to provide a larger fill opening and they do tend to be small. I am not sure what you are using for mulch but you might try Jet Spray from Profile which is easier to add through a small opening.
Based on your comments it seems that they have a nice unit and I wish them luck. I may try to email Dana later today and offer some help and suggestions that would let him promote his units for free through the hydroseeding association. Sometimes with a new business letting people know about you can be one of the hardest and most expensive tasks.