This is actualy a very good question I asked myself many times, especialy when I was in a downswing.
I read many books on poker and poker startegies, especialy on SNG's. At first I was playing a lot of SNG's because they looked very interesting to me. As I couldn't afford a big starting bankroll I had a lot of problems dealing with fish players, who called my bets with rags and hit something on the river. After a lot of upswings and downswings and a lot of time invested I realized that I'm practicaly breaking even and that I can't push to the next level, so I gave up.
Next thing I tried was NL 10 players tables. I had some up and downswings, not big ones, but I was again dealing with bad players willing to call my bets with any starting hand. I did more readings, stategies testings etc.
I slowly discovered that a lot of players suggest to play fixed limit games. I haven't considered playing them until lately, because I found them very boring. Searching for poker strategy articles I came accross PokerStrategy site less than a month ago. I registered, passed the quiz (really great stuff inthere) and imidletly started to read articles. They convinced me to give FL 10 players games a try. All I can say is: "Where a hell have I been till now?"
FL 10 players games work great for me and I'm making my constant progress. They alow me to multitable, but as for now I'm able to play just 2 tables for longer time, and 4 tables for about half an hour, maybe one hour.
Yesterday I was playing my first tournament after a long long time. It was $500 Freeroll at Mansion. I was using tournament strategy from basic SNG/Tournament articles and finished as 6th out of 2084 players.
Conclusion:
1. I'm never going back to NL games
2. I will play SNG's just as a break from regular FL games. There is just too much of swing (up and down) for my taste.
3. sometimes I will play tournaments, to give myself some variance. I have to see if yesterday's success was just a biginner luck or something more
4. I will spend most (90%) time playing FL 10 players games.
That's how I see things. Hope this will help you making your decision.