The Rans F5 rolls on the 650c wheels, like the Bacchetta Corsa. As far as I can tell, that pretty much limits you to 23mm wide tires. The Giro26 as you note has 26" rims and can take fatter or thinner tires, depending on your preference. Rans makes the Enduro at a higher price point that has 26" rims. If you hunt around you can also find the older style Rans F5 Enduro with a trussed frame that takes fat tires. http://www.angletechcycles.com has a few of those in stock.

I came very close to a dual 26" highracer and opted for my Rans V-rex for commuting and riding with my kids. The 20" front wheel and lower bottom bracket make stopping & starting much easier than the 26" front wheel. If I'd found a Giro20 for the same price as the V-Rex it would have been a tough choice.

You should also check out the Volae bikes: www.volaerecumbents.com since they're mail order. The Expedition is basically their Giro26 equivalent, the Tour has a 20" front wheel. Their bikes are on sale at www.hostelshoppe.com right now.