As a Norwegian and obsessively interested in Norwegian tartans I should tell you to go for the Romsdal tartan. The Sinclair tartan can always be aquired later, and it will always pop up a good bargain on the Sinclair tartan in the future. But from your own arguments it seems to me that you are leaning towards the Sinclair tartan. So if it's the Sincliar you really want, That's the one you should go for.
If you order the Romsdal tartan, prommise me you send a picture to me so I can post it on my sites www.kilt.no and www.tartan.no

Skål!