I would recommend that you get the best shirt you can, and get it a little looser than you might otherwise in order to learn the groove while baing able to lock out weights you can touch. As you get more and more comfortable, get tighter and tighter shirts of the same design in order to get the maximum benefit out of the time you’ve spent learning a groove.
As you are in a double-ply league, I don’t see any reason to go single-ply.
As to poly vs. denim: How is your lockout? If your lockout is well above your full-range, then go denim. If you are well balanced, or even weaker in the lockout, then go with a poly. The denim will give you huge support at the bottom, but leave you more to your own devices near the top, where a good poly will only give you great support at the bottom but it will keep supporting you all the way to lockout.
If APA rules require a closed-neck, make sure the shirt has a stretch back, if not, go with an open back.
Don’t forget with any stretch or open back shirt you will need a belt to hold it in place.
As to brands, makes and models, there is little I can say definitively. If you are a flat-back bencher go with a Fury (they do make double-ply), if you are an arched bencher an F6 or a Rage X will probably suit you a little better.
If you are worried about the learning curve, go with a Double Phenom or an Annhilator: they will be closest to your Blast, but a lot more so. You will not get as much out of them as a double Rage/Fury/F6/Denim but they will be easier to learn.
If you go for an Inzer Denim, make sure to ask for a “Radical Cut”. If the APA will allow reinforcement, get the shoulders and the neck reinforced. Don’t forget the Grid Stitching. I believe all of Karin’s denims have this stuff in?mine came with anyway. Be careful with the fed rules, some feds consider reinforcement to be an extra ply.
Generally speaking all of the manufacturers make a high quality product, don’t worry about getting shorted there. Titan will be cheaper, but you will have to wait a lot longer to get your gear (My most recent custom F6 took just short of four months from order to delivery) so if you can buy from a reseller (liftinlarge, plgearonline, etc) do so. Inzer will be faster but cost more (My Custom DD from Inzer took sixteen days from order to delivery), however a single-Titan F6 is $85 where a single-RageX is $135. Metal gear seems good, and EliteFTS’ service is second to none, but it is expensive: the single-IPF X-type is $195. The Metal shirts don’t fit me well (I have a big chest and little girly-arms) so I have no personal experience with them
Personally I use a single-ply F6 in IPF meets, and I’m about to try my first APF meet in an Inzer double-denim as described above.