Based on pure hearsay, I have been told that there is not a tremendous difference. Certainly not enough to move from Alpha's to Beta's. The logical performance progression, (Infinity wise) would be from Alpha's to Cascade's. Now with that said, once Alpha's are not longer available THEN the Beta's become the logical mid-range Infinity speaker choice. That's with Primus as the entry level, Beta as the mid level and Cascade as the high level. When considering smaller size scale I have learned that indeed the Infinity 4000 series is an excellent choice.
Here is a simple perforamance/value Infinity Systems speaker rating scheme. (Given all of these products remain available on the internet or retail market place).Performance - Full Size
Value - Full Size
Over the last 14 years I have owned the following Infinity Sytems speakers
Alpha 50, 20, 10, 37c, 1200 (Sub)
Entra Three, Two, One, Two Center, Two Sub
Entra Point Five HTS (Home Theater Speakers)
Note: My current subwoofer is a Velodyne.49Merc