Cyclocross - Sizing Assistance

Stats: 173cm tall, 78cm inseam, 65 cm arms. 68 cm bottom bracket to saddle length.

I currently ride a 54cm 2007 tricross comp. Geometry can be found here:
http://www.specialized.com/us/en/bc/SBCBkModel.jsp?spid=22302

I think it's a size too big. The seatpost is barely at the minimum height. The reach is OK, I think. I have zero drop from the saddle to the bars (because the saddle is so low).

I'm buying the Pedal Force CX2 and am torn between the 50 and 53. The geometry is here:
http://pedalforce.com/online/product_info.php?products_id=20501

The 53cm PF has the same eff, same seat post angle, and almost the same head tube angle as my tricross. The bottom bracket is 10mm higher on the PF.

When I run the online size programs I seem to get a 51cm seat tube with a 55-56cm eff. Which seems a little crazy.

I keep vacillating between the two PF sizes. I'd have a better stand over with the 50, but I'm concerned that the top tube is too short. Im worried that I'll have too little stand over on the 53 and a tiny amount of seatpost exposed, which would effect my seat to bar drop.

Anyone have any advice?

short top tube offers a opportunity for more setback , weight the rear a bit more ,
adding traction.. longer stem also gains back length lost.

Any shops around with cross racers on Staff?

Well, I decided to get the smaller frame. I just think I'm a bit too long right now, so going down a size is probably a good idea. Hopefully this won't be an expensive mistake. I doubt it. With all of the obsessing I've been doing I think the most reasonable conclusion is that I could make either size work.

Without getting into the whole handling issues of a longer stem etc, I think in the long run, it is easier to make a too-small frame fit than it is to get a too-big frame to fit. Most of my team mates that have sold relatively new bikes have done it because the frame is too big, never because it was too small. Until women's specific fits came along, my wife was constantly struggling with top tubes that were too long. You can only shorten the stem so much (believe me, I have tried).

If any one wants to start a big long 'fit thread', please see my signature, and it's reference to my opinion.

For the original posters tricross bike, I would suggest trying a stem with less rise if you feel like you have too much drop.

My thoughts are that a bike ridden in actual CX races should have fairly minimal drop from the saddle to the bars. If you set it up like a road bike (with a lot of saddle to bar drop), it will be difficult to ride CX in the bar drops over rough terrain. Due to the slower speeds of CX you dont need to be hunched low for aerodynamics, the comfort and handeling is much more important.

"short top tube offers a opportunity for more setback , weight the rear a bit more ,
adding traction.. longer stem also gains back length lost."

For actual CX racing, it is desireable to have your weight more evenly ballanced between front and rear wheels for better handeling and more floatation over soft surfaces. If traction is an issue on a climb, it is easy to shift your weight back. A more forward position is also more conducive to the the short burst of power that are needed for repeatidly accelerating out of corners. CX races are short in comparison to road races and you need a position optimized for exposive power rather than long haul comfort. Also note that if you start with a too-small frame, raising the handlebars has the effect of moving the handlebars even further back, making a short top tube effectivly even shorter than the top tube measurement would indicate.
Case in point - I started CX with a frame that was about 2cm small, I couldnt get the handlebars position high enough so had to spend too much time on the hoods rather than the drops. Switching to a larger CX frame (allowing higher bar position) that was same size as my road racing frame was much better CX position for me. Top Tube standover height is a severly overrated consideration for choosing frame size.