I think ask this just 13/14 years ago, people would have definitely said CC and not thought any different. For me, the CC seems to have taken a backwards step and the GF is the significant trophy. Wire have won the previous 2 CC finals, and yet are still ridiculed for having "no success". The GF, for me, is a bigger occasion and shows the success of a season long performance, as oppose to 5 games, which, unlike this year for Wigan having played Wire, Saints and Leeds, can sometimes be easy, 2nd team games for 2/3 of the 5. The play offs force each team to play at least 1 of the top 4 teams in the campaign to get to Old Trafford.
I'll definitely pick GF without a doubt, I've always thought it would be nice to win the CC but the GF does it for me. Also, a victory over Saints easily beats a victory over Leeds in any situation!