But this is just a frame in a shed so you could use 90x35 and don't put anything on it too heavy - or use the 120. Depending on the thickness of the ply you use for the 'roof' the batten would be enough.
But bugger all cost difference anyway.
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.