how to see "where" SQL is better than PLPGSQL

Hi all. I see an entire database, with all the stored procedures writen in plpgsql. Off course, many (if not all) of that SP are simple inserts, updates, selects and so on.

So, i want to test and show the differences between doing the same function in pgpgsql vs. plain sql.
Im getting statistics (via collectd if that matters) and doing a modified version of the pgbench tests, just using pl (and sql) functions instead of the plain query:

\setrandom delta -5000 5000
BEGIN;
SELECT pgbench_accounts_upd_pl(:delta, :aid);
SELECT get_pgbench_accounts_pl(:aid);
SELECT pgbench_tellers_upd_pl(:delta, :tid);
SELECT pgbench_branches_upd_pl(:delta, :bid);
select pgbench_history_ins_pl(:tid, :bid, :aid, :delta);
END;

At first, pgbench is showing a difference between the "pl" and de "sql" versions:

(pl.scripts own the "PL" version, sql.script owns the "SQL" version of the test)
(This is a tiny netbook, with a dual core procesor)

gherzig(at)via:~> pgbench -c 2 -C -T 300 -f pl.script -U postgres test
duration: 300 s
number of transactions actually processed: 13524
tps = 45.074960 (including connections establishing)
tps = 75.260741 (excluding connections establishing)

gherzig(at)via:~> pgbench -c 2 -C -T 300 -f sql.script -U postgres test
starting vacuum...end.
duration: 300 s
number of transactions actually processed: 15125
tps = 50.412852 (including connections establishing)
tps = 92.058245 (excluding connections establishing)

So yeah, it looks like the "SQL" version is able to do a 10% more transactions.
However, i was hoping to see anothers "efects" of using sql (perhaps less load avg in the SQL version), at the OS level.

So, finnaly, the actual question:
¿Wich signals should i monitor, in order to show that PGPLSQL uses more resources than SQL?

Thanks!
Gerardo