Well, can't remember the prices exactly but Sky3ds is more expensive

For best place to purchase skY3DS, you may want to ask someone else but gateway has a list on their site of many various places where you can buy the cart
2nd, Sky3ds works off firmware 9.4 and below, Gateway on only 3ds/2ds/xls (n3ds coming soon) 4.5 - 9.2
Sky3ds has a new cart released that fixed the game limit so now its unlimited I believe
CAn't remember which is the newer one, i believe it is the Blue (Best to ask someone else)
Gateway has homebrew support, region free and emunand, sKY3DS pretends to be a legit cart and does not have any of the mentioned features, Sky3ds everyone feels has a higher chance of being blocked out in future patches so some say it's a gamble, Gateway is thinking of various ways to access the exploit as it relies on a web page thus making it the safer option

Yes, you put microSDs in the cart with sizes of 128gb being the max supported
on sky3ds yes you can just access eshop like on a normal 3ds, on gateway however you will need to access the files you wish to download via Emunand

N3ds/xl may be getting support "soon" we don't know when, most likely this year, possibly by n3ds launch date but we don't know

Hope that helped

P.S: "Also I hear SKY3DS is rougly $120 and Gateway is $80 as well and most people prefer Gateway as the best method of running backups etc. (me included) but if you are on a firmware higher than 9.2 you have to either get Sky3ds or wait for 9.3+ support which could take anywhere from now to like a year, no one knows"

 

I think gateway is about 6$0 and the sky is about $75 at the lowest. Some retailers will offer free or minimal shipping.

The newest model for sky3ds is the blue one, not the red.

It is important to note that although the sky works on the latest version, you can't do anything if nintendo patches it in a future firmware update. Gateway can use emunand to emulate the latest version, bypassing this, but sky3ds has no tools to patch it. That's why we have 2 versions: red button and blue button. If anything happens that renders the sky3ds obsolete, you'll have to buy a new sky3ds model, if they release one. Your only choice is to disable wifi to prevent updates, which also forfeits your ability to use eshop or play online.

I bought a sky3ds, very well aware of this possibility and am more than satisfied already. Plenty of single player games to satisfy the price tag :]