Hi Gail,
i see that no one else has yet responded. I looked up both types, and noticed that with the 'Guelph Millenium' you get male plants. It doesn't say so about the 'Mary Washington improved' on the few sites i read. To avoid rampant reseeding, i sure like to plant the males. I chose both Jersey Knight and Jersey Giant, and still received about 4 -6 females out of 100+ crowns planted 5 yrs. ago. Those few females have baby asparagus popping up in different areas of my gardens, as i love those fronds with red berries for fall decor! Still, i'd choose all males if possible.
Good luck

Thanks, Gladofit. I thought I was being 'shunned' - ha ha!

Last spring (2005) we planted 75 each of Guelph Millenium and Viking KB3 (an open-pollinated, male-female variety, but more productive than Mary Washington). We didn't want to put all our eggs in one basket, and those were the two varieties offered by the nursery, Keddy's in Kentville NS.

The GM came highly recommended by a NS asparagus specialist who'd done field trials with them, and by an Ontario government website. We took a light cutting for a couple of weeks this year - this is now recommended, over the old advice to wait a year; it's supposed to stimulate node production. Great flavour, very productive.

All-male varieties are more expensive but worth it I think.
Good luck!