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:

[https://scifi.stackexchange.com/q/96054/4935][1]

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

  [1]: https://scifi.stackexchange.com/q/96054/4935