UKC

In relation to mtb components - what is Boost ( or non boost ) ? And importantly how do I tell the difference ?

( Mike - king of buying non bargains off ebay -)  

It's a hub standard. Boost hubs are wider, 148mm as opposed to 142/135 on the rear (or 157mm super boost for DH bikes) and 110mm as opposed to 100mm for the front.

Some hubs have adaptors so you can switch between boost/non boost

Because a boost rear hub is 6mm wider the cassette is 3mm more outboard, so the chainline is 3mm wider measured from the centre of the frame. Confusingly you'll see chainsets sold as normal, boost, superboost but also by chainline e.g 49mm 55mm

it essentially allows the wheel hub to be wider which gives two advantages

1. for the rear it means the chainline can be optimised for wider range cassettes as well as 1x chainrings

2. it means the spokes are set further apart making the wheel stronger and more stable

Ive ridden both boost and non and can't tell the difference, but then im not pushing my bike to the limit (well, maybe my gravel bike but thats non boost and no problems)

If youre buying a new bike then buy the bike you want and can afford, regardless of whether its boost or not.
 

For mtb, it's probably better to go with boost mainly because it's the newest standard so replacing wheels etc will be simpler. Some hubs have adapters which mean you can swap between different frames using a spacer for the disc. For a while I had two mtbs, one boost and one 142mm and swapped wheels between them using adapters where necessary though all my wheels have Hope hubs which are simple. I think DT Swiss and Stans hubs can also be easily swapped about. Don't know about Shimano or SRAM.

Though like you Ebay is my favourite bike shop and if you can find 142mm wheels and frames they'll likely be much cheaper. 

I recommend bikeradar for info.

I was just reading about this topic two days ago. 

My next bike will have boost spacing