I second the vote for Schwalbe tires. There are tons to choose from, but the Marathon series are the most durable and longest lasting. I have Marathon 420 (Greenguard) tires on my commuter and my cargo bike. I have 700x38 on the commuter and the cargo bike uses 26x2.0. It's the same tread pattern for both sizes. I chose the 420 because the center section of the tread will roll smooth on the pavement but there is enough "toothiness" to the tread to allow traction on gravel paths or light fire road/off road. There are reports of at least one guy getting 8000 miles out of a set of Marathon 420 tires. There are some minor issues regarding the weight of larger tires versus smaller tires, but it is now widely accepted that larger tires actually have less rolling resistance than smaller tires. There are some other issues that come into play like having larger tires with a more supple casing, but generally speaking you need not worry that going to a larger, more cushy tire, will slow you down. In fact it just might speed you up ever so slightly.