I’m jealous – it’s a stunning place.
I’d take the MTB, definitely.
Like others have said there are some fun trails through the Machair, and you don’t need a fatbike to ride the beaches on the West coast – the sand is pretty firm around the tide line – we managed most of the west coast like that on heavily laden MTBs with touring tyres.
If you’re coming in from Lochboisdale, head straight on at the crossroads at Dalabrog and to the end of the road – you can pick up the machair trail (through the “golf course” :-)) or get onto the beach from there.
There are a few bits of rocky coastal singletrack around Loch Sgioport on South Uist – old paths to deserted crofts etc. You need to negotiate a fair few “off piste” boggy sections to link them up though – all good fun.
Stunning scenery everywhere but I particularly liked the NW coast of North Uist – some more offroad options up there (including the only forest on the islands!), but again a bit difficult to link up – don’t let that put you off though – it’s all worth exploring.
We didn’t ride it but there is also a trail bisecting Benbecula which also looks like it may be rideable in places at least.
You can (apparently) even ride across to Vallay on a low tide if you’re feeling brave
Don’t miss the Balranald nature reserve – lovely beach – heaven on a nice day.