what conditions seem to favor each drilling. have a short[1 3/4] pin omega crux and a 4 in pin down Phaze 2. The ball motions both seem really smooth, Crux seems to roll earlier, but i have the cover at 1000 grit and let the Phaze 2 lane shine. If you had the same ball, same cover prep, would their be a big difference, and what condition would favor each drilling. Am a senior[moderate speed, lower rev] bowler. Thanks for any input in advance

Short pin balls are really good when you need optimum control mainly used for flatter tournament patterns.  You can still use short pin on league conditions when it calls for it.  But typically you use that layout when the conditions are flatter and you need to square up playing outside.  Short pin layout is more of a specialty layout and has a specific purpose.  Bowlers with high rev rates, especially two handers, will love this layout.  Typically lower rev players will struggle with this layout because they need more help with backend motion, not less. 

A pin down ball is going to get through the front part of the lane better than a short pin, due to the simple fact the pin is farther away from your axis point, and offer more backend motion while still not being uncontrollable.  This layout is more versatile in that you can use it on a much wider variety of lane conditions both league and tournament.  However, due to the smoothness of the backend, playing extremely deep angles you will typically see your carry percentage go down as the backend is not angular enough to kick out corner pins consistently.

A bowler with your specs (moderate speed, low rev rate) would flourish with balls that are medium RG (2.50-2.53) and higher diff cores (.040 and above) because the RG would help the ball glide through the front while the higher differential will get the ball revving up and help with backend motion.