I've had many Elises (still have a S1), Evora S and an Evora 410. I now have a 981 GT4. My thoughts:
Steering feel is not quite as good as the 4xx but still very good (much better than the 714 GT4 IMO)
Sound is mega and not in your face as much as the 4xx (only just though)
Turn in is probably a bit sharper on the 4xx
The GT4 COG is lower and really turns around you in the corners.
Performance is about the same. its a good torquey engine, and really flies above 4800K
The auto blip (if you turn it on) is brilliant (thought I would hate it) but when you are 'on it' it helps loads and sounds mega.
The car is fun at legal speeds, and moves around a fair bit in a good way - it will wag its tail if you want it to.
The interior finish/controls/stereo/etc and build in general is way ahead and the whole car just feels increadably well put together and just feels like you can be abused.
People will say the gearing is longer than the 4xx but in reality its not as it will rev all the way to a peaky 7,800 and hence more speed on the road. I personally don't really notice it except occasionaly pulling out of junctions.
The manual gearbox is pure joy to use and probably on the the best ever list.
Brakes are perfect and very strong when needed
The suspension is firmer than a 400 from memory I think, but excellent - it handle mid corner bumps brilliantly. Its switchable as well depending on your mood.
The 918 carbon buckets hold you very very well and really bring drama to the cabin, however you need to try first as they are not for everyone
I would only get the clubsport with roll cage if you are planning to track lots as it will introduce more cabin noise.
Minimal sound deadening, so fair bit of road noise on some surfaces. Its more pratical in terms of storage, as you have 2 boots and the fronk is very deep and takes a lot of bagage.
I love it, and just got my deposit back for the Emira. I can't see me changing it for anything sub 100K. I just got back from a 2K miles trip around the Alps (with another and some Evoras) and it just cements my opinion personally.
You need to go for an extended drive if you can, as its takes a little distance and time to get under its skin. It has great depth.
Theres probably other stuff I'm missed.