I've done N2O a few times now, it's completely different than saline...
There's air in the dispenser and it's difficult to keep really sterile so prolly a bit more risk of infection.
If you get the needle in the wrong spot you could do some damage from the pressure and have to be careful about not doing too much at once or it gets very cold...the cartridges from some brands actually have food safe/sterile on them so that's hopefully not too much of a risk...
You don't feel as full or heavy since the gas is weightless and since it goes away pretty quickly you don't get the long filled sessions like with saline but if you tie off tight enough you can get very tight and stretched...if it deflates a bit, you just need to give it another puff to stretch it to the max. You have to be patient even though it looks like you're blowing up like a balloon at first...it'll take a while for your skin to relax and let the gas permeate completely...that's when you get the best fills though
There's no seeping or blisters so you can get really tight and full without the complications...
Migration is going to happen. Tying off tight mitigates it as does going slow but once you start to max out the pressure the dispenser is capable of will almost assuredly force some gas up into your belly or into your dick...as long as the injection site is in the fatty layer just under the skin you should be fine. Word of caution here, you're going to cut off circulation to some extent to tie off good enough to prevent migration...use soft cord or something you can untie easily when it's time to stop.
As far as how fast it goes down, this will vary from person to person. In my case I'm abut 75% deflated after a few hours, 90% after a day the last 10% takes a few days to dissipate but is not really noticeable, you just hear a hollow tapping sound when you tap the inflated bits. If I got to the same size with saline it'd take me about 3 days to deflate and there would still be a noticeable bulge on the last day...
You probably won't get as big with N2O alone as you would with saline, tying off reduces the room you have and there may be less permeation but it feels really good...if you want to get real big, N2O a day after saline is the best