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Hi Derek. The 130mm at F7 (900 divided by 130) is fairly 'fast' and I would expect a fair bit of false colour on bright objects. The extra aperture of the 150 would normally create more chromatic abberation problems, but in this case it would be offset somewhat by its slower F ratio. I would imagine they'd be neck and neck in that regard.
The thing is, the 150 is a BIG scope and would definitely need something like an EQ5 minimum, as Iddon said. I used to own a Meade 150 (the AR6), very similar optically but, at F8, was a little shorter than the Saxon. Still it was big! And the supplied LXD55 mount was barely adequate. If you're serious about astrophotography, you will definitely have to reconsider the mount. Even just for visual use.
The 150 achromats perform very nicely on deep space. Nothing beats an unobstructed light path. At high powers (planets, moon, double stars etc) some can get a bit 'iffy' in terms of chromatic abberation. It can be improved quite a bit with the addition of a violet reduction filter. William Optics make a good one at a reasonable price. If you're lucky and you get a good one, you'll be surprised at how well they can perform on really good nights.
But think about that mount.
As to any advantage of the extra 20mm in the case of the 150, I'd say it'd be minimal, all things being 'equal'. However my gut tells me that, both being simple achromats of considerable aperture, and the 150 being slower (larger focal length to aperture ratio), the 150 would have the edge in overall performance.
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