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John, A good barlow will not noticeably dim the image for a given magnification, e.g., 7.5mm Plossl, and a 2x barlowed 15mm plossl should be as bright as each other. But the higher power view will be naturally dimmer as you have the same amount of light spread over a larger area.
No, a good barlow won't produce any "distortions" of the image.
The magnification factor refers to the size of things, e.g., Jupiter will have 2x the diameter when you place a 2x barlow between EP and focuser, compared with the unbarlowed view.
Chromatic aberration is not an issue with good barlows IME. Having owned the shorty-plus and the meade #140 3-element apo barlows, and three 2-element barlows that easily outperform both of these, I believe that the "apo" barlow concept is mostly a gimmick.
2" vs 1.25" is up to you, but the 2" gets a bit awkward with 1.25" EPs when you have to fumble around in the dark with the big barlow and one or two 2-to-1.25" adapters. But if you have 2" or 1.25"/2" hybrid EPs (like Stratus, Hyperion or LVW) that you definitely want to barlow, 2" barlow might be the way to go.
You can get an el cheapo GSO 1.25" for around $20 from Andrews, but chances are you will be disappointed. If you're getting it because you want higher power now but cannot afford to spend any more, then grab one, but if it's to see if you would like barlows then don't. For that I'd recommend no less than a televue barlow.
My recently acquired 1.8x 1.25" televue barlow is a very good one - I assume the newer 2x would be at least as good. Antares 1.6x is a good 2" barlow and fairly compact - I got mine from Scopestuff. The 1.25" Televue is just a little better at controlling stray light from bright objects, and has visibly better barrel blackening and lens coatings.
One tip: You can increase the power of barlows using a barrel extension. E.g., the Antares 1.6x becomes a 2x with a 2" extension, or a 2.4x with a 4" extension without any noticeable degradation in performance. So a lower power barlow is desirable IMO because you can always up the power with an extension tube, but you cannot go the other way.
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