On Interest Only Morgage. Better to Overpay or Go Interest Plus Capital

I have an outstanding mortgage of circa €100k. I currently have an interest arrangement to 2014 but am considering paying off some capital, say €1K a month.
Does anyone have any views as to whether I should just overpay on the interest only arrangement or formally switch to an interest and capital agreement.? Are they much of a muchness or is there anything to consider?

Thanks People!

If you switch to a capital and interest arrangement, you may never get the interest only option back. Something to consider.

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Maybe save the money in a high interest account in the meantime?