Hi-Lo is better
"Is it more efficient for betting purposes to count 3-7 as +1, rather 2 through 6."
Simple answer: no. You've gotten some confusing answers, above. For betting purposes, the EOR of the 2 is about 0.38, while that of the 7 is 0.28. (See Appendix D of BJA3, Table D17, p. 522.)
Uston created his count (3-7 = +1) for two reasons: He started a school in NY in the 1980s, and he wanted a count with his name on it, to rival Hi-Lo. Also, for similar purposes, he wanted counts with his name on them to put in his book, and he was urged, by Stanley Roberts, to get them created. Note that, ironically, the Uston APC was created for this purpose, but Uston himself didn't use the count that bore his name; instead, he used the Revere APC.
In any event, the 2 is more imnportant for betting purposes than the 7, so if you're going to count one as +1 and the other as 0, the 2 wins. Period.
Don