If I were in your situation I would definitely opt for a petrol strimmer, however you will still find that the decking would be have slithers of grass on it but this does depend on the skill of the user.
If you were to go down the petrol strimmer route invest in some goggles/face protection and ear protection and of course do the usual safety checks of the ground. If you have a large area that needs strimming regularly it is worth the investment or maybe see if a friend, etc has one so that you can give it a try. The difference in power is quite a shock and it may take some getting used to.
All the best.