Better if You're Full Member?

[TobiasOhawk]

Hard to say, depends if you're confident in your PC's performance and internet connection.

If you have a reasonable setup and you're still lagging unlimited will help with the fullscreen mode and no adverts, not to mention all the extras you'll be able to unlock.
Also check pirates.exe for a memory leak using task manager and the processes tab when the game is running, I get bad lag when it starts to take up more than 600,000K of RAM (roughly takes up 300,000K when first loaded) and my PC has 2 gigs.