Hi Coralee,
Eventually it always comes down to the numbers eh?
http://www.execulink.com/~lfoord/index/BettsofPEI.htm
In this database it says that it was a David Jr. Betts, who married Susanna Darby, apparently a son of David who was born 1781....

I had completely missed that tidbit before but it makes sense as you pointed out, David born 1781 would be in his 40's by 1823 when the marriage to Susanna occurs.

Although it seems logical, I can't find supporting evidence of David Jr's existence or that David Senior born 1781's wife's name was anything other than Susanna Darby.

Except this website which states...
records of the Church of England at St. Eleanor's show David baptized on May 11, 1854 at the age of 73, and he states his birthplace as "Mirarsiche.......whether David immigrated with Isaac is not known for sure. He is not in the 1827 Wallace census and his son David married a PEI native c1824; we are assuming he did.
David Jr. married Susanna Darby, they had at least eight children born at St. Eleanor's (all appear in the Church of England, Richmond{(St. Eleanor's}) baptismal records and it is believed that David Jr. died at St. Eleanor's; Susanna died in 1856 at the age of 55 at St. Eleanor's

This makes Susanna born 1801 and 20 years younger than David Sr. born 1781, so it's not impossible that he was her husband, what do you think?