Lens Recommendation

I have this posted on the RMNP form, but I wanted a little wider opinion base.

I'm hoping I can get some recommendations about a big lens purchase. I'm looking at getting a good telephoto lens (Sony/Minolta mount, 1.5 crop factor) in the area of 500-600mm. I'm currently using a Sigma 100-300 f/4 with a 1.4 TC but want something with a little more reach.

I'm wondering between:

1) A Sigma 500 f/4.5

2) Minolta 500 or 600 f/4

3) Sigma or Minolta 300mm f/2.8 with a x2 TC

I think the 300 with a TC would be a little more versatile, but I'm interested in opinions either way. The lens will mostly be for wildlife - fox, coyote, elk, pika, marmots.

Thanks in advance for your help.

If you can wait, Sony plans to introduce a new 600mm lens in August, along with the new 24mp flagship model. They just introduced a new G tele zoom lens and a new Zeiss wide angle that will cover full frame. More lenses are in the works. But, the problem will be, the 600mm will cost a fortune ( the new Zeiss 24-70mm zoom is $1,700!!!!!). Sony is really blowing it in this respect, their pro glass is way too expensive.

I agree that the 300 with the x2 tc would give you more options and will be more portable. Avoid the Sigma 500 f4.5 its massive and slow. Remember that Sigma lenses with Minolta mounts are slow compared to the same models in Nikon or Canon, so if possible, get a Minolta or Sony.

Good to see someone else using something beside a Nikon or Canon digital.

You're not kidding about Sony glass being ridiculously expensive.

I'm still leaning towards the 300 + x2 TC, unless I can find a used Minolta 600mm f/4 (anyone? anyone?).

I think that you will probably probably be very pleased with the performance of a 2.8 lens. I have a Nikon 80-200 with a x2 TC and get great results. I have had slower lenses in the past and they become a struggle with light.
Hope this helps.
Peacemaker