Similar querys, better execution time on worst execution plan

Hi all!

I have a strange behavior with this query:

SELECT c.id_contenido,p.fecha_publicacion,c.titulo_esp,c.activo,c.activo,s.label_esp as label_sbc,p.orden
,p.tapa_spc,p.tapa_cat,p.tapa_principal,p.id_publicacion,ca.label_esp as label_cat,sp.label_esp as label_spc
FROM cont_contenido c ,cont_publicacion p ,cont_sbc s ,cont_cat ca ,cont_spc sp
WHERE c.id_instalacion = 2
AND s.id_instalacion = 2
AND p.id_instalacion = 2
AND c.id_contenido = p.id_contenido
AND c.id_sbc = s.id_sbc
--AND (c.activo = 'S' or c.activo = 's')
--AND (s.activo = 'S' or s.activo = 's')
AND upper(c.activo) = 'S'
AND upper(s.activo) = 'S'
AND ca.id_instalacion = 2
AND sp.id_instalacion = 2
AND ca.id_cat = s.id_cat
AND sp.id_spc = ca.id_spc
ORDER BY sp.label_esp ,ca.label_esp ,p.orden

This is the execution plan:
Sort (cost=128.81..128.83 rows=5 width=189)
Sort Key: sp.label_esp, ca.label_esp, p.orden
-> Nested Loop (cost=0.00..128.76 rows=5 width=189)
Join Filter: ("outer".id_contenido = "inner".id_contenido)
-> Nested Loop (cost=0.00..24.70 rows=1 width=134)
Join Filter: ("inner".id_spc = "outer".id_spc)
-> Nested Loop (cost=0.00..22.46 rows=1 width=111)
-> Nested Loop (cost=0.00..6.89 rows=1 width=68)
Join Filter: ("inner".id_cat = "outer".id_cat)
-> Seq Scan on cont_sbc s (cost=0.00..4.44 rows=1 width=35)
Filter: ((id_instalacion = 2::numeric) AND (upper((activo)::text) = 'S'::text))
-> Seq Scan on cont_cat ca (cost=0.00..2.31 rows=11 width=33)
Filter: (id_instalacion = 2::numeric)
-> Index Scan using cont_cont_cont_sbc_fk_i on cont_contenido c (cost=0.00..15.56 rows=1 width=43)
Index Cond: ((c.id_instalacion = 2::numeric) AND (c.id_sbc = "outer".id_sbc))
Filter: (upper((activo)::text) = 'S'::text)
-> Seq Scan on cont_spc sp (cost=0.00..2.16 rows=6 width=23)
Filter: (id_instalacion = 2::numeric)
-> Seq Scan on cont_publicacion p (cost=0.00..98.54 rows=442 width=55)
Filter: (id_instalacion = 2::numeric)

If I replace both "uppers" with "...= 'S' or ...= 's'":

SELECT c.id_contenido,p.fecha_publicacion,c.titulo_esp,c.activo,c.activo,s.label_esp as label_sbc,p.orden
,p.tapa_spc,p.tapa_cat,p.tapa_principal,p.id_publicacion,ca.label_esp as label_cat,sp.label_esp as label_spc
FROM cont_contenido c ,cont_publicacion p ,cont_sbc s ,cont_cat ca ,cont_spc sp
WHERE c.id_instalacion = 2
AND s.id_instalacion = 2
AND p.id_instalacion = 2
AND c.id_contenido = p.id_contenido
AND c.id_sbc = s.id_sbc
AND (c.activo = 'S' or c.activo = 's')
AND (s.activo = 'S' or s.activo = 's')
AND ca.id_instalacion = 2
AND sp.id_instalacion = 2
AND ca.id_cat = s.id_cat
AND sp.id_spc = ca.id_spc
ORDER BY sp.label_esp ,ca.label_esp ,p.orden

This is the Execution plan:

Sort (cost=193.98..194.62 rows=256 width=189)
Sort Key: sp.label_esp, ca.label_esp, p.orden
-> Merge Join (cost=178.07..183.75 rows=256 width=189)
Merge Cond: ("outer".id_contenido = "inner".id_contenido)
-> Sort (cost=60.11..60.25 rows=56 width=134)
Sort Key: c.id_contenido
-> Merge Join (cost=57.31..58.50 rows=56 width=134)
Merge Cond: ("outer".id_sbc = "inner".id_sbc)
-> Sort (cost=10.60..10.64 rows=15 width=91)
Sort Key: s.id_sbc
-> Merge Join (cost=10.00..10.32 rows=15 width=91)
Merge Cond: ("outer".id_cat = "inner".id_cat)
-> Sort (cost=5.10..5.12 rows=10 width=56)
Sort Key: ca.id_cat
-> Merge Join (cost=4.74..4.94 rows=10 width=56)
Merge Cond: ("outer".id_spc = "inner".id_spc)
-> Sort (cost=2.50..2.53 rows=11 width=33)
Sort Key: ca.id_spc
-> Seq Scan on cont_cat ca (cost=0.00..2.31 rows=11 width=33)
Filter: (id_instalacion = 2::numeric)
-> Sort (cost=2.24..2.26 rows=6 width=23)
Sort Key: sp.id_spc
-> Seq Scan on cont_spc sp (cost=0.00..2.16 rows=6 width=23)
Filter: (id_instalacion = 2::numeric)
-> Sort (cost=4.90..4.96 rows=21 width=35)
Sort Key: s.id_cat
-> Seq Scan on cont_sbc s (cost=0.00..4.44 rows=21 width=35)
Filter: ((id_instalacion = 2::numeric) AND ((activo = 'S'::character varying) OR (activo = 's'::character varying)))
-> Sort (cost=46.70..46.94 rows=93 width=43)
Sort Key: c.id_sbc
-> Seq Scan on cont_contenido c (cost=0.00..43.66 rows=93 width=43)
Filter: ((id_instalacion = 2::numeric) AND ((activo = 'S'::character varying) OR (activo = 's'::character varying)))
-> Sort (cost=117.96..119.06 rows=442 width=55)
Sort Key: p.id_contenido
-> Seq Scan on cont_publicacion p (cost=0.00..98.54 rows=442 width=55)
Filter: (id_instalacion = 2::numeric)

The question is, why the query with the worst execution plan (most expensive, the second) runs faster the query with the better execution plan?
First Query: 10 runs, avg: 8 sec.
Second Query: 10 runs, avg: 1.8 sec.

I see a fail on the "best" exec plan, the rows I get are around 430, so the first EP expect only 5 rows and the second EP expect 256.

I run 7.3.2 over Solaris.
I did "vacuum full analyze" before

Thanks in advance!

Fernando.-