There are lots of threads on this topic, so for detailed information, you may want to read up on both and then decide which you think is better for you.

Here's a basic breakdown:

Flatline: Generally a little more accurate than an Apex (if it is tuned properly); doesn't shoot quite as far as an Apex; can be picky about the paint you use.

Apex: Not quite as accurate as a well tuned Flatline; a little more range than a Flatline; can be attached to any barrel with a barrel adaptor; easy to clean.

That seems to be the general concensus, but you really need to read up on both so you can decide for yourself, depending on your play style, the paint you have available, and the fields you play at.

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