What fly line #?

Hi all,

I have a Hardy Sirrus #5 fly rod, and I was wondering what fly line I should
get.

Currently using a Scientific Angler Ultra3 WF5 F line but I find it
difficult to cast.

An angler tells me that it is better to use a #6 line on a #5 rod, the bloke
in the shop said I should get a #5 line.

I am not sure what line to get and I do not want to end up buying half a
dozen lines to find the right one, so I thought I will ask the experts on
here for some advice.

Thanks.