There isn't really a difference in the meaning of these sentences, although if you use 'think' and 'guess' in other contexts, they can mean completely different things.

丁寧に説明してくれてありがとう！ 実に微妙なニュアンスの違いなんですね。参考になりました! 　:)

They both mean the same thing but using guess is more vague and carries more nuance. If you told me "I think I'll go to sleep" I'd assume you want to sleep. If you told me " I guess I'll go to sleep " then I'd start to think maybe you don't want to sleep. Another context: "Are you going to the party?" "I think so." (You want to but you are not sure if you will). "I guess so." ( You are going but you don't really want to).