We have a saying around here: The best insulin is the one that works for your cat. Some cats respond better to one "flavor" of insulin than another.
Lantus and Levemir are both depot insulin and that's where the similarities stop. Lantus is an acidic base, cultured in vats of ecoli, can sting at higher doses, some cats react to the sting even at low doses as well. It forms time released crystals under the skin.
Levemir is a neutral base, cultured in vats of brewer's yeast, no string at any level, it binds and unbinds to the albunim (sp?)
Tibbs is still early in the dance I wouldn't look to change insulin just yet. Lantus so far has the highest remission rate although mostly because Levemir is fairly new for use in felines. And usually used more in lifelong insulin dependent or high dose kitties.
Levemir also has a later onset and a later nadir normally but every cat is different.
Mel and The Fur Gang