That is a difficult question to answer and a decision you will have to make. It depends upon your long term goals for the system. The Opticon Vokal is designed to go with your Opticon 2 Mk2 speakers so it should have similar sound. I assume that you will be using a subwoofer with this system? The downside is that the Opticon Vokal is a basically a 2-way design. Not ideal if you plan to have friends over for movie viewing but ok for single user. Having the same 5 Opticon speakers for the bed layer should yield a nice surround experience.
The Rubicon Vokal is a true 3-way design which is preferred. That is a huge difference in price, though. What you will gain is better off-axis response for people sitting off to the side, and deeper bass from the centre. Will it match well with the Opticon 2? Can't say for certain as the Rubicon line is a step up from the Opticon. The exterior finish is also different, so how important is it that the 3 front speakers have the same look? If you spend the extra for the Rubicon centre, how long before you can afford to add the surround speakers?
How easy is it to sell speakers in your area? Could you try the Opticon Vokal and later upgrade to the Rubicon if it does not meet your expectations? If this is going to be a long term purchase and you do not plan to change anything for a number of years, spending extra for a better centre may be a better long term decision. We can provide information and recommendations but the choice in the end is going to be yours.