Caradhras is higher. It is a mountain (in the common sense of the word), whereas Halifirien is a hill (again in the common sense).

When the Company is in the foothills of the Misty Mountains (The Ring Goes South, in The Fellowship of the Ring). Gandalf says it will take two marches to reach the top of the Redhorn Gate pass which is far lower than the summit of Caradhras (otherwise it wouldn't be a pass). In contrast, Cirion and Eorl walk up Halifirien, hold a ceremony at the summit and walk back down in a single day (part (iii) of Cirion and Eorl in the Unfinished Tales). Notably, they start their descent as the evening is setting in, reaching their camp at the foot of the hill in time for supper. Starting a descent from a mountain in the evening would be utter madness; had the descent been long they would simply have camped at the top.

It's also worth pointing out that the Lord of the Rings explicitly states that the Beacons were in the foothills of the White Mountains, not on the tops of the mountains themselves (Minas Tirith, in the Return of the King). There is an extensive discussion of this here:

Do the beacon lighters really live on top of the mountains?

In short, Beacons on top of very high mountains would be silly, so the men of Gondor didn't build them there.

Caradhras is higher. It is a mountain (in the common sense of the word), whereas Halifirien is a hill (again in the common sense).

When the Company is in the foothills of the Misty Mountains (The Ring Goes South, in The Fellowship of the Ring). Gandalf says it will take two marches to reach the top of the Redhorn Gate pass which is far lower than the summit of Caradhras (otherwise it wouldn't be a pass). In contrast, Cirion and Eorl walk up Halifirien, hold a ceremony at the summit and walk back down in a single day (part (iii) of Cirion and Eorl in the Unfinished Tales). Notably, they start their descent as the evening is setting in, reaching their camp at the foot of the hill in time for supper. Starting a descent from a mountain in the evening would be utter madness; had the descent been long they would simply have camped at the top.

It's also worth pointing out that the Lord of the Rings explicitly states that the Beacons were in the foothills of the White Mountains, not on the tops of the mountains themselves (Minas Tirith, in the Return of the King). There is an extensive discussion of this here:

Do the beacon lighters really live on top of the mountains?

An important conclusion there is that beacons on top of very high mountains would be almost impossible for a pre-technological state to maintain.

Caradhras is a mountain. Halifirien is a hill.

Look up Amon Amwar (another name for Halifirien) in the index to the Unfinished Tales, or read parts (iii) and (iv) of Cirion and Eorl for detailed descriptions of Halifirien.

Caradhras is higher. It is a mountain (in the common sense of the word), whereas Halifirien is a hill (again in the common sense).

When the Company is in the foothills of the Misty Mountains (The Ring Goes South, in The Fellowship of the Ring). Gandalf says it will take two marches to reach the top of the Redhorn Gate pass which is far lower than the summit of Caradhras (otherwise it wouldn't be a pass). In contrast, Cirion and Eorl walk up Halifirien, hold a ceremony at the summit and walk back down in a single day (part (iii) of Cirion and Eorl in the Unfinished Tales). Notably, they start their descent as the evening is setting in, reaching their camp at the foot of the hill in time for supper. Starting a descent from a mountain in the evening would be utter madness; had the descent been long they would simply have camped at the top.

It's also worth pointing out that the Lord of the Rings explicitly states that the Beacons were in the foothills of the White Mountains, not on the tops of the mountains themselves (Minas Tirith, in the Return of the King). There is an extensive discussion of this here:

Do the beacon lighters really live on top of the mountains?

In short, Beacons on top of very high mountains would be silly, so the men of Gondor didn't build them there.