Originally Posted by
GhostRider62
Bike fit isn't as simple as M or L or 6 foot vs 6'1''. In case, Reach and STA are much more important than Stack. For instance, the horizontal distance from my BB to the hoods is 622 mm and from the nose of my saddle to the bars is 597mm. This is a 150mm stem and bars with 125mm reach on a 58 cm frame with stack and reach of 605mm and 396mm. I should be on the 61cm but even still the stem would be 130mm for the same static fit (with a flipped stem). You can force a rider onto a smaller frame without much difficulty, but is it a good idea.
If fit were so simple, we would not have so such a market for bike fitters.
Back to the OP's question, there is one very big difference between the two bikes from my perspective that deserves consideration. The BMC has a 1 degree steeper STA compared to the Giant (in my size), which effectively decreases the Reach by 10-15 mm (guess....I did not do the calculations). Weight forward and move over the crank is fashionable these days......personally, I prefer to have less weight on my hands and would opt for the Giant
Changes in the seat tube angle have NO affect on reach, since reach is the horizontal distance from a vertical line through the BB center to the top-center of the headset bearing. You're talking about moving the saddle fore/aft and stem length but forgot bar reach. All of those can change the fit.
Unless you're new to cycling, you should know what range of STA works for you and what seatpost setback you want. Reach and stack tell the whole story unless you're moving the saddle to change reach instead of getting yourself balanced over the BB. I've bought six frames in four years, based solely on reach and stack. I ride a smaller frame and they nearly all have a 74-75 degree STA. If one had a 75, I'd use a 32mm setback post instead of a 25mm. If the frame has a proprietary post and only 15mm of setback, I wouldn't buy it. I use the same saddle on all bikes and set the saddle tip 60-65mm behind the BB at the proper height first. I also use the same 80mm handlebar reach, so only the stem length might vary from a 100 to a 110.
Also, frame reach can only be compared at the same stack height. Every 10mm of increased stack reduces reach by about 3mm.