Let's see, I must say I really like the subtle mind exercise and I have no trouble getting into it and being focused, it relaxes me, there is no pressure involved and I do not have to use too much mind power.
Loving-kindness practice makes me work, it is really reaching into the depths and letting go and letting love in.
I will use both practices in my personal life to foster "mental fitness". For subtle mind, I will use it when I have endless chatter in my mind and the way I would do this, is to set aside a space for me probably on a nice pretty carpet with warm lights and some cushions and there I would spend time being quiet and practicing the subtle mind exercise.
The Loving-kindness practice would be done at the same place where I would be spend time each day thinking about the ones that I love, the ones who were not so nice to me that day, and include the one who have not be so great to me in previous times and then a larger group such as victims of domestic violence, homeless children etc and let the love flow...