droz-
The 18 by 9's = 162 square inches while the 12 x 12's = 144 sq. inches.
The 18 x 9 are 1.125 (= 1+1/8 th) times as large as the 12 x 12's but the 12 x 12's are longer with more leverage. From this point of view, I'd say it's a wash, unless someone who has had both can say differently.
The trim tabs not only trim the boat, but also add planing area to the hull and help the boat begin planing earlier and plane at lower speed. I think the difference would be moot, here, too.
Bennett trim tabs have been the Gold Standard among trim tabs for years. Tom, the technical advisor at Bennett, is a legend among long time boaters for his helpfulness and accommodation.
The Lenco electric trim tabs had some water intrusion problems initially at the shaft seals, but the manufacturer seems to have solved those problems. Salt water was particularly a problem, as were boats left (moored) in the water for long periods. Personally, I'd still prefer hydraulic tabs for moored boats and heavy salt water use, but you could take your choice and probably be OK. Note: The Bennett trim tabs on my Sea Ray have been in the water continuously at Shasta Lake for 24 years and are still working fine!
If you were going to install them yourself, the electrics would be easier, as all you have to bring through or over the transom is small diameter (comparatively) wiring. The hydraulic tubes are larger, but most of the difficulty comes when you have to install the pumps back in the transom area, down behind and under most everything else imaginable (!) Can you say: #@*&^!?*#!!! :lol:
Good Luck, more comments will follow!
Joe. :teeth :thup