How do I calculate the best return on savings, mortgage vs ISA?

BP

I have a share-to-buy property, with a 12 year mortgage of £78,000. I own 30% of the property, so pay rent on the rest. Simplifying the numbers a little, I pay:

£600/month toward the mortgage
£500/month on rent

I believe I can pay another £500/month toward some savings.

As I see it, I have three obvious options:

1. Continue the property payments as they are and put 500/m into an ISA or similar
2. Increase my monthly mortgage payments to 1100/m
3. Buy out the remaining 70% of the property, which would remove the rent, but hugely increase the mortgage because my deposit will be lower as a % of the total, then put the now £1600/month total towards it. I'd also pay stamp duty and solicitors fees on the transaction.

How can I calculate the ROI over time on each of these options?

Related: I have more than £15k liquid-ish (current + various ISAs) outside of my mortgage. Would it be prudent to put some of this toward the mortgage (regardless of which of the above options I choose)?

BP

Reading again it's possible this post is in the wrong forum - should be in the regular Mortgages forum rather than MFW. Is it possible for a mod to move it?