???
Thunder or Fragger?
Do you like to kill people? Clearing room with explosives? And basically just killing people?
Choose Fragger.
Do you like to control people? Blinding the whole room and mow them down? And basically just blinding people?
Choose Thunder.
Do you like to feed enemy some pineapples?
Then, Fragger
Do you like to make people alt+f4? Oh, wait, they cancelled this function. So I should rephrase it.
Do you like to making people say @&$?!@&@! In front of a white screen and make them rage quit?
Choose Thunder.
Do you want to bang them all?
Or
Give that asshole a second asshole?
Both have their ups and downs…I’d say Thunder is a bit harder to play but if you can place those nades well and avoid flashing yourself, he has a bit more of a reward.
What kind of pasta noodles should I cook today?
What? Its entirely up to me and totally subjective? crazy!
/sarcasm
Sorry, I just really hate it when people ask this question.
Apparently SnakekillerX really does not like cooking pasta noodles… 
I honestly don’t know why people have a problem with a player asking the community which merc they should buy. It is just a question that promotes communication between players, improving the community as a whole in the long run.
People can say “WHO CARES!”. Well I can imagine the person asking cares… at least a little… otherwise why would they ask?
Now as far as the question goes…
If you want more 1 shot kills I would choose fragger.
With Thunder you need to work a little more for the kills. Luckily he does help against bunny hoppers.
I would say pick Fragger first and then get Thunder later. In the end you will be able to get all the mercs anyway 
Thunder is easier to play imo, but both are effective in different ways. My only pickle with Thunder is that while I prefer him, I rarely play pub now and try to play 5v5 pugs as much as possible, and it seems like the US guys basically ban Thunder by default, so I try my best to play Fragger instead.
Fragger has the better weapons and better ability for an assault. Still waiting for a MK46 1 extra damage buff, which would put Thunder on par with Fragger at least weapon wise. Thunder is far more usable now than he was when he originally was released last year.
