I think if you check the fantech web site they recommend two 90* turns before going thru the roof,at least the diagram I saw showed this and it is the way mine was built. The 10" silencer must be close to 4' long and has to lay horizontally hence the two turns, but it sure is quiet. I accidentally left the fan on at about 2/3 speed and the trap door to my attic was not in place and I could not figure out why it was so hot in my house until it dawned on me the exhaust was pulling hot air from the attic. It is rated at 1266 CFM and 1060 CFM after installation.

Inline:
Pros: easier installation: no need for installer to climb out on roof
blower may be more durable as not exposed to heat/cold/rain/snow etc vs roof mount
less obtrusive external appearance if roof is visible from ground level

Roof mount:
No risk of noise from fan in attic being heard in rooms located beneath attic mounted fan
As you note, decreases need for space in attic for install

Overall, not a huge difference IMHO. Both are far superior to an internal hood mounted blower from the standpoint of noise. We have the same Fantech FKD 10XL blower and LD10 silencer as paddy does for our Modern Aire hood. It's an incredibly quiet system. We had looked at the Abbaka roof mount fan as well and I think either would be great.

Actually Paddy if you are referring to the picture on Fantech's website of a "typical installation", it is just an example they happened to give. Not a recommendation or requirement. The silencer can be mounted in any orientation, and of course the less bends the better. I confirmed this with Fantech this morning because you had me a bit scared - my installation needs to be a straight shot up to the roof.