ok, if no pros chime in,

I would get a small one. That way you have the highest percentage of retained roots and the least transplant shock. You gator bags are an interesting idea and I like it. Should be difficult to overwater or flood a sycamore especially in the hot dry summer.....How do you place them? Just plop the bag of water down on the uphill side of the tree?

I usually buy smaller stock and try to avoid the need for root surgery / shock. If the roots look fine I use the soil they were in figuring I'm never going to fertilize the tree afterwards so it may as well enjoy the fu fu potting soil until its roots displace it or soak up all the nutrients.

Good luck.

You will get different answers about bare-rooting depending on who you ask (and often depending on the climate in which their experience was gained). My personal opinion is that complete bare-rooting is both overly stressful and unnecessary. Especially in the case of smaller stock, I do remove all loose potting medium (often the root system does not occupy the entire volume of container medium) and loosen the roots. I go about half way between doing nothing and complete bare-rooting. Leaving some soil attached to the roots can go a long way towards reducing fine root loss and significant transplant shock. The type of plant/roots also makes a difference. Trees that naturally develop a pronounced taproot or fewer and thicker roots can be quite different from those that have fibrous root systems.

My view on tree gators is that they are an expensive and fancy way to do something that can often be done for much less money and effort. Even in cases where tapwater is not available, a 5-gallon bucket with a small hole in the bottom seems much more practical to me. I think the main advantage of tree gators is the financial gain they bring to their suppliers. I have yet to see a good argument for their necessity in the majority of cases.

Here is a link that might be useful: Planting a Tree or Shrub