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Bike fit question: does exposed seat post length matter?

I'm looking to get a winter frameset and want to match my current bike's geo (an M/L Giant Defy Advanced) as much as possible. Basic requirements are clearance for 28mm with guards and rack/light mounts. Oh and it needs to be cheap.

That's led me to the Spa Cycles Elan but the thing with that bike is that whilst the reach is similar the stack on my expected frame size (either a 54 or 56) is much, much higher (getting on for 20mm according to I've already got the bars slammed on the Giant so I think the front end will be too high for me.

Now, if I drop to a 52 in the Elan I get almost identical stack and reach, but with a 20mm shorter seat tube, which will mean much more exposed seat post. Actual top tube length of the Elan is also 20mm shorter. Obviously the Giant is already a compact geo, and the Elan is too, although to a lesser extent I think. I don't have that much seat post currently exposed on the Giant so I don't think it will look odd, but will the extra seat post length affect fit, given that the stack and reach are basically identical? I know that the higher the saddle is the greater the actual saddle to bars measurement becomes but that's tunable to a certain extent with different offset posts. 

For what it's worth, I'm 5'11" with proportionally shorter legs (30" inseam) and I've usually found a nominal Large frame suits me. That's what virtually every manufacturer suggests for me based on height (exception being Merida and Giant where both suggest their M/L size). So I'm distinctly puzzled an Elan 52 would seem to be the ideal option. Harrogate's a 9 hour roundtrip so I can't go and try one.

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