Panasonic 100-300 or ......?

I completely agree. In the end, only you know what you want to spend your money on. Let me give you a few thoughts to consider while you agonize over this:

• Having one system means that every lens works on all bodies
• The question you need to ask yourself is whether you need the theoretical improvements that the D5200 brings over the G5
• Are you done growing with the m4/3 system? I.e. are your skills limiting you, or is the sensor limiting you? For most people, the answer is the former, yours truly included.
• Are you an extreme high ISO shooter? The G5 is good to 3200 at least. The D5200 is about a stop better, give or take.
• Are you an action shooter? If you were, you would already own a dSLR. But if you plan to become one, then shifting to a dSLR system makes sense. But the D5200 is not the strongest entry ... you should consider waiting for the D7000 replacement.

Lots to think about. Don't make a snap decision. The other person who mentioned rentals has a pretty good idea. If you plan on dumping the m4/3 system for APS-C then buying more expensive lenses for m4/3 does not make a lot of sense. The D5200 and 70-300VR loses about 25% of reach on the Panny solution, but you get steadier shots because of the ability to plant to your eye (which is why I suggested the G5 in that role with the 100-300) ... so you could grab any of these kits and dump all m4/3:

D5200 with:

18-300 (adequate but far from stellar IQ)

18-55VR and 55-300VR (standard kit and good quality)

18-55VR and Sigma 50-500 OS HSM (more expensive but even better reach than the 100-300)

Lots of options ... lots of bucks.

Good luck ...

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http://kimletkeman.blogspot.com