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100mm travel, 15mm through axle both with pop lock.
Whats the difference, would I realistically notice it?
Not much AFAIK:
Weight - Reba about 100g heavier is the main one, although someone may be along in a mo who would be able to explain the differences with their damping, the SID has some fancy other stuff to do with a dig valve and rapid rebound (personal knowledge of function - zero).
How that manifests on the trails and how the fork feels to you, I can't be of help.
So, I'll go with weight for the time being. Would you notice an extra 100grams?
Reba and SID share the same chassis. The SID in general has better (more functions) and lighter internals.
The XX SID gets a carbon steerer too and a hydraulic lockout which takes away some of the adjustment.
If you want a light fork with good damping and affordable price go for Reba.
If you want a slighter lighter fork with better damping go for SID RCT3.
If you are a racer and have the money for it, go for SID XX.
The SRAM website does a reasonable job explaining what the various features do, click on the technologies link. You can decide if you want them or not.