They are made differently for starters Sam - and work utterly differently - though they both do the same job, for those for whom they work optimally.
You would need to do a basal test to establish if you are getting the best out of any basal insulin, it's the only way you can find out and on DAFNE or another carb-counting course, you do have to learn how to do that, and do it! It isn't actually hard. See
http://www.diabetes-support.org.uk/info/?page_id=120
Nobody knows how long Lantus or Levemir act either! each person is different. It was pushed originally that it lasted more or less exactly 24 hours and it released itself more or less absolutely evenly. It doesn't do either of those things for most people, in fact ! Lasts from 18 hours to 36 or any point in between, and has a peak after approx. 5 hours after which it starts to tail off.
It just depends exactly when you happen to need whatever dose of basal insulin you need.
Copepod - I may have been terribly unlucky, but I had completely irrational and no-warning hypos on Humulin I and S and I'm lucky to have survived the road accidents I had on the M42. In fact I never had a scratch, or when I practically crashed two different motorbikes by going hypo on the pillion. For me Humulin is the invention of the Devil I'm afraid ! But if it works for somebody, I'm pleased, probably LOL
Lantus stings me like hell and I sort of landed up where I don't want to touch any insulin with Eli Lilly's name on it ever again if I can help it. However I am mindful of the fact that they got insulin off the ground in the first place, for which all of us should be properly grateful and I am ! Insulin stinging and going hypo actually mean I am still alive.
Which is a GOOD thing to me !