I cannot say that one is better than the other. Both have their benefits which help us in our spiritual progress. Doing sadhana (meaning practicing roop dhyan) develops more attachment to Radha Krishna and gradually reduces the fickleness of the mind. This is important for fueling our progress.

Doing seva enhances this by improving our feelings of humbleness, surrender and dedication to Hari & Guru. It also gives us a different way of engaging our mind in devotional thoughts, since one may not be able to sit all day in meditation.

A sadhak should strike a balance between both sadhana and seva. One should not think that because they have a lot of seva to do that it is unnecessary for them to do sadhana; and one who enjoys doing sadhana can further enrich their spiritual development  if they start doing some seva.