Many people who dream of debuting want to debut under a group. Why? Because:

1. If it's a big group, you don't need back-up dancer
2. If it's a big group, no one can see you mistakes, most of the time
3. Becuase you have other people to talk to, to enjoy life with
4. There are many inspirations like SNSD, SHINee, Super Junior etc. who debuted as a group

I bet that's all the reasosn you need for wanting to debut under a group, feel free to add more. But not many people want to debut as a solo singer like BoA, duo like Fly To The Sky or a trio (dunno who's a trio). Because:

1. Everyone can see you made a mistake
2. You'd HAVE to get everything in the dance/song right to not make a mistake in all of your performances and do hard work
3. You have no one to talk to
4. Vocals are stronger when singing with others
5. You would always need back-up dancers for every dance, when you would actually want people you know to dance with you
6. It's more comfortable to be dancing with people you know
7. People will compare you to other solo singers like BoA
8. You won't be as popular as groups

But, what about debuting as a group? Let's look at the negatives:

1. You'll be compared to other groups like SNSD, SuJu, SHINee and 2NE1 even if you are a trainee, or a grou palready debuted for 1 one year, doesn't matter, you're gonna be compared to any other group out there
2. They say concepts are running out for new groups, so all new groups are going for is innocent which is getting old (I recommend that girl groups start doing a boyish image, but everyone just LOVES staying innocent and girly >.>)
3. There WILL be a start of at least one fanwar/'____ is better' comment on a new MV
4. Fans WILL start to fight
5. There are some trolls, bashers etc. even critics that will hate your group
There are many more negatives

So what about the positives of beign a solo artist?:

1. There would be less fanwars since there are more groups than solo artists
2. The hard work pays off and you won't make a mistake on stage if you work hard
3. If you get close to an artist, it'll be much more safer because once people start shipping group couple, there will be more hate on both groups. And more people (than usual) would think that you two would be cute together
4. If you really do have a closer friendship with someone then more people would call it a true couple
5. You get more screen time ^^
6. If you do a cute concept like normal, more people would actually think it's cuter rather than when groups do it
7. You can only hear you voice (duh, because you're the only one singing) and more people will think it's beautiful
8. Debuting in a big company gives you an advantage

Feel free to add more. But honestly, I think debuting as a solo artist would be much more affective on your career than debuting in a group.

This is my fantasy:

I want to get into SM Entertainment, yes, specifically SME because it's the best company in my opinion, even if you have to go through harsh training, because you need to work for it. So, I'm a trainee at SME, I debut as a solo artist. I don't want an Asian tour just after a few albums. I plan about 5 albums before I go into the American market, but not going to do any y concept. I'm going to show them how beautiful my voice is, just like Beyonce.
Once the whole world knows me, THEN I will go on a WORLD tour, see international fans and try to communicate with my fans often. I will do everything and be the best I can be, work hard, put 100% into my work. Earn the money I work for and at the same time enjoy life. Interact with my fans, care for them like a family etc. An, well dominate the world let the world know me for who I am, and not care hwo gets in my way, no ones gonna stop me!

If I were to debut in a group:

Train hard with them! Become a family! Get to know them better! Love each other! Become best friends, possibly sisters with them. I want to debut at the start of the year, make a full-length album (because, NO ONE comes out with a full-length album anymore, besides Super Junior, all they've been doing is full-length albums and it's better than just a mini-album or a single) with my group. Work hard, make at least 3 MVs for the album songs. Then for the repackaged album I want to compose at least one song.
Then after promotions, I want to do a few concerts around South Korea then I want to come back with ANOTHER full-length album, that's right, in the same year, two albums in one year! It's a challenge! I really like challnges, but I want to prove that my group is very hard-working. While being in the group, I want to compose most of our songs and choreograph at least 10 of our dances.                  
Also, I want to be in a big group like SNSD or Super Junior. Because, the more people, the more love we share amongst each other. And I want to do many reality shows just like Super Junior.

I honestly think that debuting as a solo artist is better. What do you want to debut as if you had the chance to?