I'll definitely go for heads.

I think that I will go with hEADS.

I’d sure do the heads if my site is fairly new and no too much authority yet in that niche else i’d go tails if that isn’t an issue.

Tony

Heads to get on page 1 rank above 5..tails to follow up .u could get both on page 1 with rank of 5 or more.
But if only doing one definately heads

Based on historical experience and acute observation. heads wins

From the information given, I would pick heads but ultimately I would have to take into account the actual construction of the keyword.

I would pick either. I like a combination of keywords. Some with a higher QSR some lower. Some with high traffic and some low traffic. I use keyword phrases that range in long tail keywords and some low hanging fruit. Heads or tails but here I would pick tails if I had to choose only one.

Heads all the way