I've playing a little less than 2 months & am ftp. I have a decent Arena team (thanks to good advice here), and on campaign I finished (& Galek can solo everything in) brutal but it's going to be a while till I can get far in nightmare. So my priority right now is doing better in dungeons. Here's my current top 7:

Galek 60; High Khatun 50, Seeker 50, Zargala 50 (my arena team), also War Mother 50, Basilisk 50, War Priest 50.

Options for my next 50 (after I bring Zargala to 60):

Apothecary, Kael, Coldheart.

I have some more good elites (Miscreated Monster, Frostbringer, etc.), but my current book situation strongly leans me towards going with a rare, and obviously the above 3 are among the best Rares. I'm leaning toward Coldheart. I feel like I need some more offensive versatility & Coldheart provides that. Kael is kind of duplicative of Galek (though I may eventually 6 star him just so I can use one of them for campaign and one for arena, as campaign gearing is sub-optimal for Arena.) Apothecary is awesome but I feel like I have speed buffs sort of covered with High Khatun. All that aside, Coldheart's strength is dungeons anyway.

Thoughts?

Edit - one more thing - as a ftp player, obviously gems are a scarce resource, but I have over 1,400 saved up - I'm wondering if (assuming that I choose Coldheart) I should spend 800 on immediately buying her masteries. Or should I use them on Galek, since he (and soon Zargala) are the only heroes who could immediately get Giant Slayer?

Not to highjack my own thread, but that bring up another point. Given how hard it is to max out masteries, and how important Giant Slayer is, is there an argument that Gems (for ftp players) should mainly be used for masteries?