If available to you I would defo advise to take the 0% credit card option. You'll have total control of your finances and ticket rather than having to deal with an intermediary.

There are more than you might think - CommuterClub place an order through a third party who issue a TfL Oystercard / National Rail ticket funded by a loan taken out on your behalf with Ratesetter. With the best will in the world, this arrangement is suboptimal when anything goes awry with either finance or ticket. CC seem to handle customer issues fairly but it can take time due to all the moving parts.

Contrary to some other posters, financing is technically via an unsecured personal loan. I'm not qualified to comment on whether that's better or worse on your credit file than a credit card balance.

A few general pieces of advice on season tickets:

1. Buy for a start date of Jan 1st 2018 or before to avoid 3%+ fare increase from Jan 2nd
2. Make sure your ticket covers all the zones you'll pass through i.e. include zone 1 even if your regular commute only passes through one Z1 station (thanks Shoreditch High Street!)
3. Annual beats all other tickets for cost of regular commuting hands down unless you take 4+ weeks off work in a single block (teachers?)
4. Make sure your Gold Card discount gets loaded onto your Oystercard at a Tube ticket office (good luck finding one!) - 1/3 discount on off-peak PAYG fares beyond your zones of cover
5. If the bulk of your regular commute is on a National Rail service (that includes the Overground on certain routes) absolutely check out that as an alternative to Oyster. You will almost certainly save money, even including non-commuting travel (see previous point - you'll still be eligible for discounted off-peak TfL fares with any annual ticket)

PM me if you need route-specific advice :-)