There have been threads written about this, but the short version may be gotten from this post, on GPS Passion:
"I placed an order for the Ultra, but then decided to cancel the order because I wasn't sure of the neccesity for the improved accuracy. This was the response I got from
Semsons:
"Thank you for your email. Don't worry, we think you have made a good
choice. Holux CF GPS Ultra is designed for field work where extra sensitivity is worth the extra dollar. However, with extra sensitivity, the trade-off is slower response time. If your main purpose is in-city navigation, we believe Holux CF GPS is better.
Regards,
Sales Team,
www.semsons.com"
Hope this helps. If you join gpspassion.com, $10, you can get 10% off of the $241 bundle to boot.