An acoustic archtop guitar (the traditional ones designed to be played unplugged) are composed of a solid top. The tone is generally referred to as a bit brighter/brasher than a flat top. Apparently the mids and the highs are supposed to stand out.
Most modern archtop guitars are not really designed to be acoustically loud, but come with pickups and are played through an amp. These types typically give a warmer, "jazzy" tone than a solid body electric (but depends heavily on the pickups and amps.. eg: some punk rock guitars are archtops).
In terms of ukulele, there appears to be 3 VERY BROAD categories:
-relatively inexpensive acoustic archtops with laminate tops, such as the Kala Jazz Tenor. It mostly sounds like a mellow acoustic uke, with an advantage of being a bit less prone to feedback as an acoustic-electric (due to heavier build and smaller tone holes)
http://www.youtube.com/watch?v=b5dqcRpOiac
-Fully electric steel string electrics, such as the Kamoa electric. These sound like archtop electric guitars, through an amp.
https://www.youtube.com/watch?v=-O0lTAB5ups
-Really expensive solid-top acoustic archtop ukes, such as the Koolau or the D'angelico. These are usually quite loud and great sounding ukes. Hard to compare it to anything, as they are.. well... unique. It is what it is. A solid top archtop acoustic with nylon strings (making its construction differ from archtop acoustic guitars, which use steel acoustic strings).
eg:
http://www.theukulelesite.com/shop-by/brand/d-angelico/d-angelico-archtop-red.html
http://www.theukulelesite.com/d-angelico-sunburst-archtop-package.html
http://www.youtube.com/watch?v=3siprzb0tKA
You will just have to listen to some sound samples and judge with your ears. To me, the D'angelico's sound warm and mid-rangey and the Koolau spruce top one sounds very bright and open.
With so much variety and experimentation in what an "archtop ukulele" is, I daresay every archtop ukulele is completely different from one another. Cannot generalise anything.