Nikon D3100 - Peleng Fisheye vs Sigma Wide-angle?

I've got a D3100 which I'm planning on getting a new lens for. Having had it for a while, I know all of the functions inside out, now only using Manual, Aperture, and Shutter priority - I'm a fairly decent enthusiast is what I'm saying really. Sometime I plan to upgrade my camera, but I don't think my parents would appreciate me selling my camera AND forking out more money for a new one, I'll have to wait a few more years for that…

At the moment I have the kit lens (18-55 vr), the 35mm f1.8, and the 55-300 f4.5-5.6. I wasn't planning on getting a new lens but I have another trip coming up soon (with every holiday get a new lens is my motto!). With about £300 ($450) to spend, what do you recommend for a trip to Venice or Rome, to take architectural pics. I'm thinking…

Sigma 10-20mm f4-5.6
Pros: autofocus, super wide angle
Cons: I have a program that will stitch my 18mm shots to make a wider shot so may not be necessary and apparently the lens is cheaply made

Or the…

Peleng 8mm fisheye
Pros: I've always wanted a fisheye, crazy effect, solidly made (apparently), really wide (so could potentially remove the bulge and use as a wide angle), cheaper so I could buy the SB-400 too
Cons: manual focus, vignetting, quality of individual lenses varies massively (apparently), and is it a bit of a novelty effect?

Before you ask the Nikon 10.5mm fisheye and Nikon 10-24mm wide angle are out of my price range!

Other recommendations welcome and thanks in advance!

I'd aways choose an ultra wide angle over a fisheye lens. The latter is too much of a special applications/gimmick lens. Also, if you have concerns about the effect of an UWA lens being available through software, then the same should apply to fisheyes as well. You could take some shots, perhaps even taken with an UWA, stitch them together and have some fisheye filter applied to it.

The Sigma won't be terribly dinky - it should be more solidly built than any of the lenses that you already own. Build quality is a very relative thing.
But if you're looking for build quality and an ultra wide angle, consider the Tokina 11-16mm/2.8.It won't autofocus on your camera, but it's built like a tank. Most of the time you'll have it set to manual focus anyway, using the hyperfocal distance. The image quality also is excellent.