Lowell makes some good suggestions and I own this lens and TC combo myself. The lens is a reasonable price but the TC's are selling for over $400 used these days if you can even find one. I also own the Bigma and although it's a great lens, it's not cheap at about $1000.00
Here's what I would suggest faced with the same choices.
If the budget is really tight then the 70-300 macro Sigma is the best bet. I did an hour of shooting with a new borrowed one about a month ago (haven't seen a lens I could resist testing). This is a very good lens with sharp images and excellent IQ. The 1:2 macro feature is a real plus since you can get a good distance away from the subject and get a very good close up. Good all around lens.
If you can afford to take it up a notch, then I'd get the Sigma APO 135-400mm F4.5-5.6 DG seen here on Sigmas web site:
Sigma - Lenses but can be had here at a much better price:
Sigma | 135-400mm f/4.5-5.6 APO DG Aspherical AF Lens | 727109
This lens has a great range and should be fast enough and long enough for almost any use. If you've seen Tom Lusk or Marc Langille's shots, they both shoot a lot with the FA*300mm f4.5 and sometimes with a AF 1.7 TC. So you can get great results with a "slower" lens as long as the lens is a good one. Someone a while ago asked Tom what settings he used with his FA*300mm and he answered something like " I don't know, I just know how the lens reacts and take the shot" He still shoots seagulls just to stay sharp. Practice with any camera/ lens combo can greatly improve your end results. So if the 70-300 is all you can afford, it will give you great results as long as you practice.
IMO it's simply not worth spending the money on a "it'll do till I can afford..." lens. You will be disappointed and wind up ultimately spending more in total to get what you want. I've learned the hard way that it's worth the wait and saving to get the best you can afford. So skip the manual focus off brand lenses. They are cheap and have the range but you'll be looking for a replacement a week after you get it.