Translation:I am going to a restaurant on Wednesday.
Both are correct and used interchangeably but can suggest different (very marginally different) things in the right context.
'I am' is present and current action (you are actively doing what you say you are doing)
'I will be' is present but not current action (you say you will but your action to do so has yet started.)
Both are correct. The only difference is what you want to emphasize. I'll explain through examples: When using に: 友だち: いつレストランに行きますか。 私: 水よう日にレストランに行きます。 (We're emphasizing WHEN we're going to the restaurant) When using は: 友だち: 水よう日はどこに行きますか。 私: 水よう日は、 レストランに行きます。 (Friend asked WHERE we will be going on Wednesday, so the answer was literally: As for/speaking about Wednesday, i'm going to the restaurant.) I'm not a pro, but I've been hearing a lot of people from my college making drama about the usage of particles in japanese, but in everyday japanese, especially youth really don't pay much attention to that. Sometimes youngsters tend to avoid using particles, cuz it's complicated (duh) :D
(Not a native speaker so I could be wrong)
I think the て + います form is more for what you are doing currently.
"I am working" as in you are doing it currently, uses て + います
"I am working on Sunday" as in you will do it, then the regular -ます verb form is used.
I hope that makes sense. That's how I understood it.