Well, as long as your doing the same amount of work, it should build your strength the same way, right? The problem is that unless you have a powermeter, it's sometimes tough to determine what your fluid meter power is relative to speed.

FYI, there are correlation charts out there somewhere to figure out the speed of a certain trainer to power.

For the record, one of my resolutions this year was to do the 'easy' workouts indoors so that I would theoretically be less tempted to go faster. YMMV.