I have a 9mm shield and am thinking about stepping it up to a 40... how do you find the recoil and shoot ability between the two.
I've never owned a Shield. The pic is of my M&P Compact 9mm. I've owned one about since they came out. I've never felt the need to go to a Shield feeling like the increase in concealability would not be offset by the decrease in capacity and shootability. I like the recoil spread out over a wider surface in double-stack grip frames. So I can't speak to the Shield even though my gun is pretty much a Shield with more firepower.
I've never owned either a Shield of M&P Compact in .40. I've got a regular M&P in 40 and personally, don't find the recoil to be much worse than a 9...but the one I'm comparing it to specifically, is a smaller gun. Neither has objectionable recoil. Compared to say, a typical full size 1911...I'd rather shoot any of the M&P variants. I like the ergonomics better than a Glock.
When I bought my SIG P938, I laid out a bunch of pistols at the gunstore and compared them side-by-side for size and ergonomics sans actually shooting them. The 938 won. The pistols included a Shield. The only pistol in the same size-range was a Kel Tec and I've already been down that road and don't want to go back. IIRC though, this was before the P365 came out. I'd take a hard look at it if I was doing this over.
Back to the .40...the main reason I've got one is I had a lot of .40 ammo. For some reason during the last great ammo shortage, .40 ammo was relatively easy to get. 9mm and 45 ACP around here, was wiped out. Another advantage of the .40 IMO is that it's easier to get top-quality self defense ammo for it than the 9. IMO the 9 in its top loads is about equal to the 40. The 40 has a lot of loads that are just as good to a bit better than the very top 9mm loads.
So to summarize, if I were doing what you're doing and really wanted it to be in 40, I'd pick the wider-bodied Smith over the Shield.
One thing I WOULDN'T do is get any of the M&P variants with a safety. No need for it.