The F500 is basically the F1 with a few more features.
The difference between the LMII and F1 is basically size and inherent voicing (and about $200 in the US). The F1 is a class D power amp versus the class A/B of the LMII. So, the F1 is smaller (same footprint, but about half the height), and has a quicker, tighter, slightly less 'warm' tone. It has a somewhat tighter low end than the LMII, and more aggressive upper mids, and a bit more hi fi top end. The F1 and LMII have exactly the same preamp and controls. You would not 'hate one and love the other' since they have a lot in common. However, most have a preference of one over the other, depending if they want it a little more warm and fat versus quick and tight.
The F500 (it's not available in the US until March) is basically an F1 with a few more features (semi-parametric mids versus the fixed mids of the F1/LMII, a more fully featured DI with level control, and a mute switch). It will, I'm sure, cost a bit more than the F1, and quite a bit more than the LMII.
The SA450 is basically an LMII with some of the additional features of the F500 in a larger box than the LMII(although no mute switch).
So, if you like it very clean and tight, and want it as small as you can get it, the F1 (or the F500 if the extra features are important to you) would be the way to go. If, on the other hand, you are looking for a bit more fat and warm, and are on a tight budget, and want it small, but not 'micro', then the LMII would be the way to go. Finally, if you dig the tone of the LMII and don't mind a little more size to get a few more features, the SA450 might be your thing.
Hope that makes sense!