Answers

If you off to scotland in September it would be worth bearing in mind that it may rain during your trip, in which case a tent that pitches the flysheet first would be advantageous. I think Andy Stay covered all the main pros and cons of the two tent types. Semi geodesics tend to have more of an angled tent roof than tunnel tents as they taper back from the front to back, wheres a two (or more) poled tunnel tent has more of a flat roof, and thus creates a bit more of a constant internal height.

How much were you looking to spend on a tent? As this can determine the quality, weight and also pack size of the tent. If you dont mind paying the price two decent semi-geodesic tents are the Terra Nova Voyager (and Voyager Superlight); rrp about £300, and the Marmot Grid (£250). The Hilleberg Nallo 2 GT is a great Two person tunnel tent - i've recently used it in Knoydart and it performed well, although a tunnel tent wasn't ideal because we were wildcamping.

If you do decide to go for an inner-pitch first tent it may be worth buying either grangers or nikwax tent and gear proof. All it does is adds a bit of water repellancy to the inner which is useful if pitching in wet weather. I've done this to a couple of tents in the past and it has worked.

Horses for courses.  Straightforward tunnel tents tend to be a bit simpler to erect and also many of them go up fly first (can be an advantage if its normal british weather); they also tend to have slightly more internal volume as a result of steep end walls.  They tend to be cheaper.

Geodisics, using a set of mutually supporting poles, tend to be far more stable in wind and much less reliant on guying out to give them shape.

The hybrids give a wide variety of pole configuration; I use a Solar 1 which is just a single pole tent as well as a Solar 2 which I suppose is a 'semi-geodisic'  as it has two poles which brace each other.

I, personally, would not go for a tunnel for a backpacking trip except where I was fairly happy I'd be in sheltered locations; I don't even think I've got one!