I have subscribed to Bowling this Month since the very beginning. I find it to be very informative on just about every subject related to being or coaching a bowler. That includes the physical game, techniques, practice, and the mental game. Unfortunately since they have gone to online, the number of "new" articles has decreased. Often they reprint an article from years past, and list in the title what issue it was take from. They always keep up with the latest equipment and give fairly sound reviews, although after a while, even they seem to get quite repetitive.
If you haven't read their articles before, I would definitely recommend Bowling this Month. If you have been a long time subscriber, I would be 50/50 on whether you will get your money's worth.
I think Bowler's Journal is still monthly. For me, I find this magazine to be much more oriented to Bowling Proprietors, and those interested in the politics of bowling. They do have ball reviews, and maybe one article on drills or practice tips, but it isn't really geared to self help. The Bowler's Journal has an online publication in addition to the magazine.
Just my 2 cents, other's mileage may vary.