Hold The Line advanced/optional rules 11.1-11.3 are the same in Clash for a Continent. They deal with attacker morale checks, rally and elite units, respectively.
Clash 11.4 refers to Indians and gives them a -2 vs forts and towns. Not mentioned in HTL.
HTL 11.4 & 11.5 are the same as Clash 11.5 & 11.6, dealing with increased artillery range on hills and reduced effect firing into forest, respectively.
Clash option 11.7 giving an attacker a +1 firing on units in waterways is a standard rule in HTL
Clash 11.8 and HTL 11.6 both deal with leaders. Clash simply has leaders add one extra die when attacking. HTL rule makes a few leaders better by adding more dice, taking more hits to eliminate or both. In HTL terms all Clash leaders are 1/1.
Clash 11.9 (flank/rear attacks) is the same as HTL 11.14.
Clash 11.10 (attacker advance) is the same as HTL 11.7
Clash 11.11 (force march) is the same as HTL 11.8 & 11.9 together.
Clash 11.12 (easier hill movement) is the same as HTL 11.10
Clash 11.13 (dragoon retreat before combat) is the same as HTL 11.13.
Clash 11.14 and 11.15 are the same as HTL 11.12 and 11.13 and have to do with changing the Command Action Points for play balance.
Finally, HTL includes two optional rules not mentioned in Clash.
The first is HTL 11.15 which only applies to Long Island and Brandywine scenarios. reducing the range of units.
The second is an optional rule in the French & Indian expansion that makes Indians and Rangers deadlier in close assaults in the woods (4 dice).
Other differences in the rules:
In Clash for a Continent victory is only checked at the end of a turn, in HTL it's checked at the end of each player turn.
In HTL leaders are a little less vulnerable to FIRE COMBAT. No matter how many 1's are rolled, only 1 confirmation die is rolled to see if the leader is actually hit. In Clash one confirmation die roll is made for EVERY 1 rolled in the fire attack. In both games a confirmation roll is made for every original 1 during Close Combat.
In HTL nothing is said about what happens when a leader is caught alone in a hex by an enemy unit. In Clash a leader caught alone is eliminated. I think this is just an oversight in HTL and players should use the same rule as in Clash for a Continent.