Robert, the both actually put out about the same flow. The 1200's a notorious for running hot so yu might want to just mod the 900. The parts I make require no tools and no modification to your 900. Just disassemble yours, insert the impeller/propeller mod. attach the cowl and mounting bracket (use 3 large MJ suction cups) and plug it in. $30 for everything but the pump. Add $20 for a new pump.
I also did a Penguin 1140 (runningin my tank now) will do an AquaClear 301 next week, and an Eheim 300 when the guy gets it to me to look at. I have the parts to make you a mod kit but the pumps I have are already sold with others on the way that as of today are sold too. One 900 will make your 29 boil dude! I kid you not, ask Ryan about the movement in my 120 from one. 2 in his 240 will give his some good flow. We may have to hotrod his 2 eventually to see what we can actually get out of them.