The new Sonic game has a lot of similarities with Mario Galaxy... ie. game design and gameplay. 

Do you think that Nintendo is trying to replace Galaxy with the Lost World? Maybe this is the reason why we do not have Galaxy 3 now and instead we have the 3D world....? 

I Don't think the game is on that level and I think ninteno is smart enough to know that.  The game looks great and should be a good exclusive for the Wii u but its no replacement for a real 3D Mario game.

Sonic games are generally short, Mario games, especially the 3D ones, are known for having lots of levels and content.
Lost World still looks solo IMO, must buy for me.

No. Mario Galaxy was a breakthrough game, one that was like nothing before it. Sonic looks like a Sonic version of Galaxy, and thus is not Wii U's version of Galaxy. However, it does look like a very well executed game that ads gameplay elements not seen in Galaxy. It's faster and looks fresh.

It takes up 8 gb of space.

Nah.

Galaxy is done anyway

I think Lost World will be really good, but I doubt it will be a masterpiece like Galaxy.

Not at all. It could be a great game, don't get me wrong, but part of the greatness that was Super Mario Galaxy (and number 2) was that you never knew what could come next, it just felt that new and exciting with each world. With the new Sonic game, you can already see much of the resemblance to the Galaxy games.
That said, it does look like a lot of fun, and I'll definitely buy it once I have a Wii-U.

Sonic lost world looks great, but it wont be a Mario Galaxy. Sonic LW will be a solid addition to the Wii U line up and another game that might entice people to the system.

I have a feeling Nintendo opted for Mario 3D world first due to quicker dev time and to get the game out there quickly. 3D Mario games take time and im sure Nintendo have one in the works.

3D World is very pretty.

From what i have seen, Lost World seems to have a lot in common with the Galaxy games, but they will also play out differently and given the context, the slow pace Mario game works better in that type of game than a Sonic game.

Playing a Mario game with the universe as background did wonders for the game. Sonic Lost World doesn`t have that kind of feeling.