First of all, in roma-ji, the correct is DOUMO and ARIGATOU. (We don't have "L" for roma-ji and in hiragana doumo is どうも and arigatou is ありがとう.)
どうも and ありがとう are incomparable. ありがとう is "thank you". どうも is an adverb and it has many meanings/ nuance depending on the sentence or the word which is used with it, in this case, it's like "very much" of "thank you very much" though it's not the literal translation. (It's not the informal form by itself.)
The basic and common translations are:
casual / informal form
ありがとう = Thank you / Thanks
どうもありがとう = Thank you very much / so much / Thanks a lot
formal form
ありがとうございます = Thank you
どうもありがとうございます = Thank you very much / so much
I hope this helps you.