Depends on the conditions.

Second bike is a real difference maker at this point in the season, worth several places easy, which is not the case for your hypothetical 17 v 21 lb bike choice.
That been said, there’s a lot of earlier races where you don’t need a second bike at all, so you could be riding round like a boss on your 2grand bike.

Triton bikes have a SuperX 105 up for £1400 atm, which seems like a fantastic price.

2 bikes.Your 17lb bike will weight 21lbs at the end of a muddy race and none of the gears will work anymore.

Only a 4lb weight increase? I’m sure I’ve had that hanging from my mech!!

More important than 2 bikes is a pit b!tch to clean the bikes between laps, otherwise it’s a pointless exercise in expenditure

How many bikes do you really need for one single ride?

http://www.islabikes.co.uk/info-hub/one-bike-cross

I race one bike in North Wales league (the rules) and north west (allowed 2). For me not sure benefit of 2nd bike is worth the hassle (cleaning afterwards!) with no one in pits with jet wash, ie only one change. Even then carting water around seems like a big job. Tend to think differently after early exit with snapped hanger though!

If you are at sharp end will be frustrating if your competitors are getting clean bikes. Otherwise all in same boat anyway. But need to ride differently to get bike to end more often than not.

Agree with comments on tubs making big difference, especially something like fmb supermud.

I think a light bike in cx is nice though, especially when on shoulder!