Comfort is very subjective and we always recommend that you try before your buy. There are no universal standards of firmness in common usage in the UK. A mattress that is too firm leaves parts of the body unsupported, creating pressure points on the parts of the body that are in contact with the mattress. A soft mattress may feel very comfy initially, but a mattress that is too soft will allow the lower back to sink into it too far, throwing the spine out of alignment.
We do not give our beds a firmness rating because of the above, therefore, we cannot categorise the Utopia for you.
The Tara and Janna will be very similar in feel, but as you intend to use a bedstead, then we would recommend the Tara as this has been specifically designed to complement them.