Welcome to BobcatForum Tommy.
I haven't seen any of onlineattachments products before. Just making my comments from what I see on their site. Looks like they are able to do 7” think material because the blade is exposed forward. Their attachment also has 4 blades vs. everyone elses 2 blades. I don't like the way the hydraulic motor is exposed and also looks like the hoses are mounted straight out the top of the motor. This may not be bad, but it looks suseptible to damage.
I do like the fact that they include a safety door specific to your machine. I wonder what type of safety door it is? Does this cutter have a rear roller like the Bobcat Brushcat? The roller on the Brushcat allows you to follow terrain fairly easy. This works well when mowing a grassy field.
Do you have a high flow machine? The Tree Slasher recommends 22-30 GPM. Your S220 should have 20.7 GPM with standard flow and 30.9 GPM at high flow. Your machine may run it, but you may lose some performance in dense grass and brush or bigger trees.
Personally, I would like to see the craftmanship of a product before I spend that much money. Hopefully you can see one in person or get some good feedback from someone here who has owned one. If you only need to cut 4” or smaller material, I have had good luck with my Bobcat Brushcat. Brush Wolf and Ammbusher also look like they make a good rotary cutter. Loftness looks like they make some good flail mowers for brush and the Timber Ax for 6” - 12” material. The Timber Ax would need high flow and is pretty expensive (about $20,000)