From my experience, two things here:

1) Legendaries aren't always the better than Epics.

2) Depends on who else you have and where you need to progress.  Sicia is great at what she does: Cause Damage. With her HP Burn, extra turn for applying HP burn to everyone, and detonating HP burn, she does Massive damage to bosses and waves alike.  If you don't already have a HP burn Champ that is useful pretty much everywhere, I'd go with Sicia.  On the flip side, if you need TM control, Deacon is fantastic for that and can also be used in almost all content.  

Both are great at what they are designed for.  Just that they are designed for different things.  If you take a screenshot of your roster and upload it, and let us know how far you've progressed in the dungeons and CB, you'll get better advice.

Impossible to answer in isolation. The two do completely different things.

I have 6* Artak for the damage. Of course Sicia is in my plan, but i am thinking if it wasn't be helpful I would have 6* Artak as damage dealer and Deacon as support. I am a Little afraid if artak + Sicia on 6* won't give me predicted development.

Plus My deacon has fully improved skills and sicia don't because all my legendary books.i put into Artak.

Artak and Sicia aren't completely overlapping. They can work together quite well if tuned properly, since Sicia places and then activates burns, and then Artak can do the same, and they can alternate per wave.

EG - if you were doing Drag20, you could have on wave1, Sicia open with A3 -> A2, and then Artak open with A2 and disable A3. Then, on wave2, you can have Sicia open with A1 and Artak open with A3.

Also, Deacon can easily remain at 5*, he doesn't benefit hugely from being 6* except in the ability to get a banner.

I don't want to make new topić so I ask in this one. I am thinking about blessing. Everybody give Deacon a commanding presence but I am thinking about giving him cruelty because of duchess lilitu. I believe she should have intimidating presence. What do you think?

Personally I'd run Commanding on Deacon and something with more utility on Duchess.