is one design better than the other?

top swing is designed to fit full sus frames with limited space for a front mech. Typically the swingarm sits where the mech clamp should go for a conventional front mech on a lot of FS frames.

The top swing mechs generally wear out faster and are weaker than conventional mechs. I've had play in the rivits far faster on top swing mechs than conventional mechs.

Conventional also tends to collect less crud than top swing.

Conventional it is then, and as a bonus the conventional ones seem to be cheaper too. I need to replace the 10 year old XT one on my s-works so just need to decide between XT and XTR now (although I'm struggling to justify XTR at over twice the price of XT!)

Conventional ones last longer, shift better and weigh less.
And they look better, if that flips your biscuit.

[Edit] whoops.. too slow! Go for xt over xtr. The difference being a cast cage versus a pressd one and a tiny difference in weight.

I had to fit a conventional to my Nicolai as the pivot was in the way, and yes they look much better than conventional.