I realized today that there are two types of one inch webbing, tubular and flat. Are there significant differences between them? Do they both hold knots equally well? Thanks.

Tubular is generally a bit stronger, more abrasion resistant and is what you should use for most climbing situations. Both tublular and flat would work however and hold knots equally as well.

You should use tubular webbing, it is more abrasion resistant, will more likely "roll" if pulled along an edge, and is plenty strong for climbing. Without trying to confuse you, if you are given the option between military-spec and climb-spec tubular webbing, the climb-spec will probably hold knots better.

fear: I have to disagree with you in regards to strength. To the best of my knowledge, flat webbing is stronger than tubular, but not used in climbing for the aforementioned reasons.

Thanks for the replies. J

what's the plan for the 1" webbing?

Well it goes like this. I was buying a few things at EMS and thought I would get some 1" tubular webbing for anchor building. When I got up front to pay I realized the girl had given me flat webbing not tubular. I bought it any way, and figured I'd find a use for it one way or the other.
As things stand looks like it may become a backyard slackline.