yep nothing wrong with either of those as far as my limited knowledge goes!
I often use the stationary bike in intervals. Or for example last night i was going for 20 mins at an intense level but one i could maintain then randomly throwing in burst of 30 seconds increased intensity but never dipping below my original pace when "resting". When i used to run 3-4 miles i would try and complete in as short as time as possible.
The only problem i could see with randomly throwing in intervals and sprints is its easy to cheat yourself , with the best will in the world you will still rest more and sprint less intensely than with set rest periods.
But iv found variety is the key. To stop boredom if nothing else