I own the Kohler 29 EFI and the Briggs 35. The Kohler 29's sip fuel - between 1.0 and 1.2 gph when I checked in late spring and I have had readings as low as 0.9. 95% of the time it has more than enough power but if the grass is really heavy, it does bog. In my opinion, I would not run a catcher on a 29. I'd say it's underpowered for that task. It's been a great engine for everyday mowing and runs all day easily on a tank of fuel. As for the 35, when I checked fuel consumption in the spring at around 100 hours, she was burning between 1.8 and 2.0 gph. You'd almost be empty at lunch. Here in the later part of summer, I've been measuring fuel usage on the 35 again and the results have been much better. With the thinner grass of summer and running standard lift blades, I've gotten a reading of 1.63 gph over the last 34 hours. I've had readings as low as 1.53 in the summer. In the spring, I've had readings as high as 2.4gph.
Which engine to choose really does depend on the end user and their needs. If all you'll do is mow, I'd go with the 29 EFI Kohler. But if you're planning on bagging, you'll want the 35. Both the 29 EFI and the Briggs have been outstanding for me and I would recommend both.