And at L2, we'll never be able to upgrade it or even service if something were to go wrong before end of scheduled mission. Something on the moon would be much more accessible for upgrades or basic servicing missions. Even with 2 week on/2 week off schedule, it would be so useful. During those 2 weeks off, it would be charging its batteries. Never would it suffer from cloudy nights.
OTOH, a trip to the moon requires about 10x more fuel than going to L2. Although I suppose a moon telescope could synergize nicely with other lunar activities
Edit: I think I misread the table I was looking it, it's more like 2x or 3x instead of 10x, and that assumes a start from LEO
