Series X: More Power, More Storage, also got disc drive
Graphically unless you have a 4k/60-120hz Monitor, or plan to buy one in the next year or 2, Its not beneficial realistically
Series S: No limitation, its next gen still, only difference is the lower Graphics Power at 1440p and less storage, whilst also being fully digital, which is not even standardised yet, its mostly still 1080p, as standard, but its the better option if your not fully arsed on graphics/dont have/plan to have the monitor/TV that takes advantage of the X
Also a question of cost, £250 + Realistically needing External HDD, or £450