The same, "我是很强的”sounds not natural, it's grammatically right though. "我很强的”might be better.

基本意思一样。没有区别。 这句话省去一些内容。应该说： 我（在体育、数学等方面）很强。

I'ts better to use 我很强 with one-syllable adjectives.

both are natural, and basically, they have the same meaning. but the latter contains a kind of tone of explanation.