I sure as heck don't wish I'd chosen differently. Both were on the table, and I chose the 10.1MP 1.3x 1D3. I do miss the reach a little bit, but I'm looking for an excuse to get a 400 f/2.8 anyway. I just need to watch the budget doghouse... I might stepping-stone to a 300 f/2.8 first, but I really don't think it's enough reach for me.
Get a 300 2.8 plus kenko 1.4x extender. I think you will be quite pleased with results! Even a 2x will provide great results
I use a kenko 1.4 with my 300 f4 and I love it! I do notice a slight iq loss but there is less of a loss on the 2.8 version