Thanks for your comment.
Something that can be useful in determining which family was which is to look at names. For example, if you look at Christian & Henry's parents' names, their siblings and their children - how do they compare to the names of your John Brice and Dorothy Herd's children? If you can find the other John Brice and Joan Drake's children - how do they compare? The names might be so common as to not give you any clues but it's worth a look in case there's a pattern.
After considering this I took a look at a family tree (see my posting in response to DEB) descending from Joan Drake ( & John Brice) and compared it with that descending from Dorothy Herd ( & John Brice).
Here are some facts that perhaps someone can use to throw light on this, and I'll add my reflections at bottom.
The children of Joan Drake (& John Brice) were thus:
Susanna Brice b. 1748 - died 1758 [aged 10]
Mary Brice b. about 1749 no info
Priscilla Brice b. 1750 died 1758 [aged 8]
John Brice b. 1752 died 1758 [ aged 6 ]
Thomas Brice b. 1753 no info
Elizabeth Brice b. 1754 died 1755 less than 1 yrs old
Edward Brice b.1756 survived, married "Letitia" and the forbear of a great family.
William Brice b. 1757 died 1758 [aged 6 months] (thus four children died in 1758)
John Brice b.1759 [no info]
All were born at Sampford Peverell in Devon.
The children of Dorothy Herd (? 'Heard') and John Brice of Cruwys Morchard were thus:
William Brice b. 1755 in Devon - a CAPTAIN of own ship. Southhampton & Guernsey.
Sarah / Sara Brice b. 1759
Anna / ?"Nanny" Brice b. 1762
Ephraim Brice b. 1764
Richard Brice b. 1765 born Cheriton Fitzpaine. Created brickworks in Guernsey.
George Brice b. 1767
Thomas Brice b. 1770
Dorothy ("Dolly") Brice b. 1772 (married a Mr. Wood)
Austice Brice b.? (married a Mr. James)
James Brice b.? no info
John Brice b.? no info
Apparently they were all born and baptised in Cheriton Fitzpaine in Devon although later hailed from Cruwys Morchard, "and sometimes of Southampton".
I found that the Herd / Heard name was relatively plentiful in Cheriton Fitzpaine and so I think their father John Brice must have married a Dorothy Herd from there and lived there.
Now in regard to your point, Darwin, about checking the congruence of names of descendants, I have looked up these descendants, as above, and it seems that the Joan Drake famliy (rather than the Dorothy Herd family) fits more with being descended from Henry Brice and Christian Rowe:
Henry Brice himself, b.1690's married Christian Rowe on 17 June 1718.
The children of Henry Brice and Christian Rowe are as follows:
1. Henry Brice b. 1719
2. John Brice b. 1722 (this is the one that two conflicting trees indentify as theirs)
3. Alexander Brice b. 1724 (died)
4. Alexander Brice b. 1727
5. Richard Brice b. 1729
6. William Brice b.1731
Siblings of Henry Brice (to compare with the names given to the grandchildren):
1. Thomas Brice b. 1695 married a Pricilla Bennett
2. Edward Brice b. 1701 married a Joah Bryant
3. John Brice b. 1704
4. Alexander Brice b. 1706
Henry Brice's father was an Edward Brice, b. 1656, in Uffculme, Devon
Henry Brice's grandfather was an Alexaner Brice, b. 1637, at Holcombe Rogus, Devon.
Among Henry Brice's siblings the name of their father Edward and grandfather Alexander is continued.
Henry and Christian seem to have named their children after (1) Henry himself and (2) Henry's grandfather.
Interestingly neither among Joan Drake's children nor among Dorothy Herd's children is there a Henry, or an Alexander, but in the Joan Drake children there is an Edward (Henry's father's name).
That fits, and so that gives one tick to the Joan Drake family.
Most notably among Joan Drake's children there is a "Priscilla". Not such a common name, and Henry Brice's brother Thomas's wife was Priscilla.
This is another tick for the Joan Drake family, since her husband John Brice was the nephew of Thomas Brice who married a Priscilla.
So I am inclined from this to have doubts it being the John Brice who married Dorothy Herd.
I appreciate your thoughts and comments and please also note the info I posted in my reply to Deb.
Will.