First, welcome to Nitro Express!

I am in a similar situation with my little Greener 310 double except mine needs a reline due to crummy bores.
I would say, pick a twist rate which will work with the bullet you expect to use most often. That is, if you have a choice of twist rates with your liner supplier (let us know if you do btw).
That way, you are set to use it as you would like.

Now, the one thing keeping me from relining my little double is that I am unsure of how well regulation will remain and whether a lined double can be re-regulated with lined barrels. I haven't seen an article on this subject so cannot be confident that it works that well.