It may be too simplistic to say they’re just two different types of basals. The actual insulin proteins are physically identical (among all insulins), but it’s the [chemistry] buffer that affects the time required to release the insulin. Buffer chemistries between basals all vary, as do the way those chemistries interact with your body chemistry.
Hence, the paradox that biochemists face in achieving a “flatter and curve” is that human biology is neither flat nor consistent, so it’s simply not possible to create a set of chemistries that reacts the same all the time.
Consider those who use a pump and happen to have the flattest and best BG control. That is, just using a pump doesn’t give you flat BG curves. To get that, you have to vary basal rates programmatically in the pump to match your biology. This may range from .5u/hr to up to 2u/hr over a 24-hr period, depending on individual metabolisms. And even those calcs change for events like holidays and vacations, work trips, school schedules, etc. These experiences make ones body more or less sensitive to insulin, but also to carbohydrate metabolism, etc. Then one must take into account physically active days vs. less active days, and so on.
In short, optimal control is not possible unless you’re willing to do a combination of self-mitigation of variations in your life (which can be easier or harder for some over others) and stricter attention to bolusing with either insulin or carbs as needed to smooth out those unexpected curves resulting from metabolic variations in your body.
Personally, I’m happy to artificially restrict my daily activities to favor “predictable routine” of activities, carb intake, and other events. But that still leaves a lot of wiggle room in metabolic reactivity to insulin.
I don’t like the pump primarily because the level of work needed to manage the device doesn’t offset the benefit received when compared to the relatively minimal work associated with a basal insulin.
So, when picking which basal insulin is right for any individual, it may certainly be that Tresiba has a flatter curve (and last longer?) than Lantus under some conditions for some people, but I’ve personally found “long and flat” is not necessarily “good” because it requires you to be static in your lifestyle all the time. If you vary anything, that long-lasting Tresiba is almost assuredly not going to perform how you need it for at least a few days.
Although basals vary among individuals, I can speak for myself (though I’ve found many others echoing the same observations) that Tresiba will peak with optimal performance between 5-10 hours after injection. Here, my bolus calcs is about 1u/20g. After 10 hrs, the insulin carb ratio increases to 1u/40, which will go to about 24-30 hours. To truly “flatten” it out for me, I had to break it up into two doses: 7am and 2:30pm – the latter was necessary because if it were later than that, my BGs would crash during the night. Any variation on that would require lots of boluses and glucose tablets to keep things stable if my activities and eating didn’t match what Tresiba was doing.
With T1D, the best control is not the result of how well you do when you’re in your routine, it’s how well you do when you’re not. Once that BG roller coaster starts, it’s a nightmare to get off of it, and can throw you off for a long time. Tresiba causes more roller coasters for me than Lantus ever did, primarily because Lantus didn’t last as long. I could start afresh each day, and not worry about what I did 1,2 or 3 days ago.
My personal experience over the past 45 years with diabetes has taught me that the “optimal” basal insulin is one that works for about 12 hours because life presents those unexpected curve balls, where your routine will be disrupted. I don’t want to be handcuffed to an insulin that will require 3-4 days to readjust when something different happens.