[Kunama]

Hi Alan, it depends on what you are trying to accomplish.

The 150/750 is an F5 scope (the focal ratio) the 1200 is an F8, for visual use the the F8 is better if you have a mount that is capable of holding the bigger heavier scope steady. The longer focal ratio is caused by the lens elements bending the light less, that is the lenses are less curved, so you will see less of what is called chromatic aberration (CA). It will still be there but less than the F5 scope. CA is in all refractors to some degree, it shows as a coloured halo around the brighter objects such as planets Venus, Jupiter as well as the moon. Some people find it abhorrent, others don't mind having it as to be rid of it involves the investment of many more $$$$ in apochromatic scopes of the highest quality. Even with brands like Takahashi, there is CA on all but the very top models.

The actual size of the image you see is a factor of focal length and eyepiece focal length so in either scope what you see depends on which eyepiece you use. (the shorter the eyepiece fl the more mag it gives)

For any given eyepiece the F8 scope will give more magnification, the F5 scope will show a wider field of view. Everything is a compromise.

As for what to buy? I would sincerely recommend you don't buy either of the 2 until you have had a chance to look through some more scopes, there are plenty of opportunities around Sydney to go to astro gatherings and the people will be glad to show their scopes.