Is buying iphone 5s now a good decision?

I like it because it's handy but I'm afraid that it's gona outdate soon. Or should I go for 6 or 6s

The iPhone 5s is better in my opinion, it's a lot sturdier, looks better, less likely to smash when dropped and you can't bend it of course unlike the poorly made iPhone 6. Although there's a new iPhone 5se being released next month which is quite similar to the 5s if you want to look into that.

Put it this way, in 2 years there will still be alot of people using 5S. If you like the bigger screen, go with 6S though.

Apple is expected to release a 4" iPhone next month, if your reason was because of the physical size.

If not, then its ok for this time still. Assuming that 4" iPhone comes out next month they may stop making it at that point, but it should still get updates for at least a year.