TP vs Ignite on mid
Personally I usually take TP in mid. I started taking TP as Xerath, cause you dont want to get close enough to ignite alot of the time, it seemed like a more useful skill.
Playing with that for a while got me taking TP on all my mids now though.
I feel the increased map presence (Faster back to lane from a Back, gank potential, jump to a single lane pusher to save a tower or to your own pusher to take a tower) is worth more overall and to the team. It has many more uses the ignite does, and TP's can help the whole team more then an ignite does.
As far as ignite, I dont find myself in as many situations where an ignite actually secures the kill. Sometimes yeah it does, but alot more its just a supplement to an all in attempt or sometimes a fail'd ignite. Usually if im getting a kill, its going to happen one way or another. If I dont get the kill, even one ignite woulda given me, I'm just as happy sending them back and pushing lane or getting ahead/catching up on CS.
Just my opinion
I take Flash and Teleport/Cleanse/Exhaust both mid and top. Never ignite.
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but get countered hard then screw taking Ignite take teleport instead hit 6 and roam / get ahead other ways, then go back after you are ahead of your mid laner and kill the person who no longer counters you because you got fed.
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as an example but 