For production class, I'd basically narrow your choices down to either the Glock 34, S&W M&P 9L or 9Pro, and the Springfield XDM 5.25.
Barrel length means longer sight radius, which is good. The Glock 34 comes in at 5.3" which is about as large as you can hop to fit into the Production rules. The XDM 5.25 is, as the name implies, 5.25", and the S&W M&P 9L comes in at 5".
The Glock has a huge following and is a great choice. The XDM is a relatively new offering in this size and hasn't been tested a lot, but initial reports are encouraging. The 5" M&P models also have been gaining in popularity.
Personally, I shoot an M&P 9L and am liking it so far. My main reason for choosing it was that it was available for a minimal premium over a standard M&P (whereas a Glock 34 costs a fair bit more than a 17), and frankly, Glocks are incredibly common and I just don't want to shoot what everyone else shoots
.
One possible benefit of the M&P versions: because they are specifically marketed as "competition" guns, the Glock 34 and XDM 5.25 are not Production legal in actual IPSC matches, whereas the 5" M&P's are still Production legal in IPSC. The vast majority of US-based shooters will only shoot USPSA and never IPSC, so that probably doesn't matter much, but I figured I'd mention it.