What are the big differences? Mechanically, they appear to be very similar, with the same makes of engine and transmission.
Also, does the fire suppressant still smell like washing up liquid (as in the 180)?
Things like the transmission or engine type make next to no difference to drivers. Generically speaking, the main points covered in traction training are:
1. Rules and regs, non-standard procedures (eg: speed restrictions - do they comply with MU/SP/HST differentials, any location specific details).
2. Cab layout
3. Below solebar arrangements
4. Interior equipment locations
5. Fault finding
6. Train failure arrangements
Followed by supervised train handling, especially if the unit has significant differences (eg: CPBC when drivers have only ever used seperate handles for power/brake, variable brake when drivers have only used stepped brake etc).
The big points that take a lot of time to cover are things like faults and failures and rules that apply to these. Whilst the rule book covers some catch all examples, there are plenty of traction specific quirks/exceptions.
In terms of handling, 20 hours may be overkill for some, but if there are drivers who have only driven sprinters for some years, the handling characteristics are going to be somewhat different and its almost as much about unlearning things that might be unhelpful (eg: muscle memory of how much you move your hand for a certain brake step might equate to too much/too little brake - for a perfect example of how important and how critical this can be, see the RAIB report for the collision at Neville Hill).
As someone who has no experience of 175s and no knowledge of the GWR program for training, I'm sure others can give a more comprehensive overview - this is just the simple summary.