If you are only doing lake sailing I'd consider the long shaft adequate (used one for some time). We wanted elect. start and wanted to get the controls up higher to be in reach better so replaced the Honda long shaft with a Tohatsu 9.8 extra-long shaft. (only their 6 HP Sailpro, which doesn't have elect start and the 9.8 come in an extra-long shaft version.
http://purplesagetradingpost.com/sumner/macgregor/outside-index.html
I mounted it a little higher so now the prop is in the water at a depth between a long shaft and extra-long shaft but closer to a long shaft. It worked fine in the ocean most of the time on previous trips but on the Bahama trip I got into worst conditions and it would come out of the water at times for a couple seconds. If I went back there or to the ocean I think I'd drop it back down to the extra-long shaft depth.'
So if ocean sailing is going to happen I'd look for the extra-long shaft myself.
The other option to think about is electric start vs. pull start. We had pull start with the Honda and it worked fine for a while as I could pull start it but Ruth couldn't. Then on Lake Powell we got into a bad storm situation...
http://purplesagetradingpost.com/sumner/MacgregorTrips-3-Powell09/09-10-9-Powell-09.html
... and I fractured and screwed up my shoulder and couldn't pull start the outboard for 3 days and neither could Ruth so we sat in a remote side canyon. That was how we came to the conclusion that we needed electric start for safety purposes. If you sail with someone who can start the outboard if you were to go overboard or be injured then I might consider the 6 HP sailpro or an equivalent but again if sailing on the ocean the extra thrust of the larger prop on the 9-10 HP can be welcomed at times,
Sumner
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1300 miles to The Bahamas and Back in the Mac...
Endeavour 37 Mods...
MacGregor 26-S Mods...
Mac Trips to Utah, Idaho, Canada, Florida, Bahamas