Premiership for sure.
CL is a bit of a lottery, if you ask me. You need luck more than anything else to win it. Many people considered Barca worthy winners last season, whereas teams like Liverpool (5th best in their own league that season) and Porto (half decent team from a relatively uncompetitive league) have also fluked it recently. Unequal local league pressures and schedules, some with winter breaks, unequal grounds etc make the competition uneven to begin with. I don't even want to begin on 4th-placed (sometimes 5th) teams qualifying for a top-tier "champions'" competition. Whatever true prestige it had has been slowly whittled away by the money-making schemes of the footballing powers.
But you don't fluke the Premier League title. 38 games against local teams waiting week after week to outfight, outrun, out-tackle, out-kick you just because you're the top dog. Derby matches, bitter inter-city rivalries that make players run their lungs out for the bragging rights, or simply to "knock you off you f**king perch". And with the considerable resources from TV rights, there are no real walkovers anymore. No one, and I mean no one, in his right mind will dispute the legitimacy or argue against the league table come the end of the season. (Note: Wenger is not always in the right mind).
For that, the Premier League title comes first.