Look at it this way. These focal lengths are the focal lengths for 35mm film (or sensor in the case of full frame dSLR). 50mm is considered 'normal' in that format. In truth, normal for us is closer to 35mm (lens) but that's topic for different discussion. Here we'll just go with 50mm. The 50-200 will give 4x magnification if you want to think of it that way. The 70-300 will give you 6x (from 50mm).
I never had the Sigma but did have the Tamron (DI LD) version of the lens. It offered 1:2 macro and I would actually recommend it over the Sigma based on my own experience only.
The best way I can show the difference in magnification (macro range) is this..
This was taken at macro 300mm with the tamron.
From the same position, this was done at 180mm macro (with the tamron)
This is non macro at 300mm
You could take the same shot with the 200mm but it would be a wider shot and to get That framing, you would have to crop your photo.
260mm
70mm
What the 300mm buys you over 200mm is the ability to stay back. It does come at a cost though. The longer the zoom, the tighter the crop (you see in the viewfinder), the more difficult it is to hold things steady. Even with Shake reduction, you'll find yourself having to dial faster shutter speeds to get things held in place.
The Tamron can be had for about the same price or less as the sigma. I mentioned CA in my other post and due to lack of experience with the Sigma, cannot give a direct compare between the two lenses. The Tamron is notorious for CA and PF in high contrast situations. It can be corrected in post processing but it may be a deal breaker for you (the bird shot is a corrected one). I am also not 100% certain that the Sigma is 1:4 macro (and not something better like 1:2). There are 100s of threads here comparing the two lenses, each has it's fans and naysayers. Both would probably serve you well and if you can 86 the DA-L 50-200 and get one of the 70-300s instead for about the same money, I would go for that option.
Now, This is a decent way to go to get Macro on an all in one lens but to really do macro, you'll eventually find yourself wanting a dedicated macro lens which can be pretty expensive. There are other ways of doing it, a Raynox 150 or 250 that will fit your 18-55 is a recommended way to go around here. Those are basically magnifying glasses that fit on the front of the lens. I mention all that, because you mentioned Macro, and by most accounts, the Pentax DA-L 55-300 leaves both the sigma and tamron laying in the dust for image quality (just to confuse you more
).
I'm sure others will sound off but I'll leave it with my original statement. When I had the 50-200, 200mm was Never enough when I wanted Zoom.