They give the same 34% discount, but the 16-25 railcard would discount the whole Worthing-Bristol journey (and not just the part of the journey in the network railcard area). And if you travel in July or August you are not hit by the £12 minimum fare before 10am. With flexible tickets, and splits at Salisbury and Warminster, and a 16-25 railcard, trainsplit.com suggests a cost of £29.40 for journeys leaving Worthing after 9am on a weekday in July or August (or after 10am on any other weekday or anytime at weekends).