amehta Loc: Boston

I know I'm late to the party but here's my $0.02.

It seems that you actually have 3 things to decide:
1. upDATE your body
2. upGRADE your body to full-frame
3. upgrade your lens

Sensor technology improved a lot until around 2012, now it's settled down a little. So updating your 50D (fall 2008) to the 70D (fall 2013, $1200) would have a significant improvement in quality.

Upgrading to the 5D MkIII (spring 2012, $3500) is a much bigger expense. Most would say the 70D is closer to the 5DIII in IQ than it is to the 50D, technological improvement over time is a bigger factor than sensor size.

The option that would be nice but you don't have is to a 7DII, because it is as lost as the Nikon D400. Eveyone gets to keep waiting.

The last thing to consider is upgrading the lens. Do you have the Tamron 18-270mm F/3.5-6.3 Di II VC PZD? The DPReview says "Autofocus is slower than its competitors, however, and image quality is weak at the the telephoto end." It's good for what it is, but it's not as good as you can get. Will you notice that for most pictures? Probably not. Will you notice it on a Costa Rica birding trip? Very likely.

The 70D, 17-40mm f/4 L, and 70-200 f/4L IS is still less than the 5D MkIII body. If you decide to add a full-frame body in the future, the lens will continue to excel. And I don't think you'll ever notice the focal length gap from 40-70mm.