$400 sounds like a great deal on a 8.8 installed with an Artec Truss. The axle itself is often around $400, and the Artec Truss just under that.
You may also need an adapter to connect it to the drive shaft, and potentially different brake line adapters.
Most likely ends up being much cheaper than a Super Dana 35.
But do you have a slip-yoke eliminator? I think having that helps with the offset of the 8.8. I already had mine due to my lift, but you may want to factor that into your cost if you don't. Your profile says 4" suspension, so I'm hoping you have one. If not...I'd recommend it.
Do you have disc or drum brakes on your Dana 35? What does the 8.8 have?
I got the nice quick disc brake upgrade when I installed my 8.8.
The C-clip elimination isn't as "important" if you have disc brakes.
I like that the 8.8 offers a lot more selection on types of differential traction devices than the Super Dana 35. However, if the Super Dana 35 has the type of locker you want, then that's great.
Me, personally, I went with the Ford 8.8 over a Super Dana 35. And I had to pay $400 for the 8.8 axle itself, plus the Artec Truss. But part of my decision was factored into preferring an Eaton TruTrac over a locker due to winter driving and budget. True lockers perform better in most other scenarios, so you can't always make a decision based on what someone else did, as their reasons for doing so may be different.
I wanted to find a Dana 44 as my #1 choice, but it was insanely difficult to find in my area, unless I wanted to pay top price at ECGS.