Why IsNull is twice slow as coalesce (same query)?

We met a strange situation on SQL Server 2008 (SP1) - 10.0.2531.0 (X64) - Win2008 SP2 (X64).

Here is a one heavy query:

select t1.id, t2.id 
from t1, t2
where 
     t1.id = t2.ext_id
     and isnull(t1.vchCol1, 'Null') = isnull(t2.vchCol1, 'Null')
     and isnull(t1.vchCol2, 'Null') = isnull(t2.vchCol2, 'Null')
     .... and about 10 more comparisons with Isnull

UPD: All columns in comparison (except IDs) are varchar(~30...200)
T1 is ~130mln rows, T2 is ~300k rows.

These query on rather big Dev server run ~5 hours - this is slow, but what we can do?

And while we investigated possible ways of optimisation - we found, that changing "isnull" to "coalesce" in query above gives double performance gain - and query now run for ~2 hours

UPD: When we remove all ISNULL checks and use just t1.vchCol1 = t2.vchCol1 the query finishes after 40mins.

Question is: Is this known behavior and we should avoid using IsNull everywhere?