I bought the game some weeks ago and got a HOTAS (the Logitech G940) ...yesterday I ordered the TrackIR, I think its 50-50, doesnt matter what you get you'll find something missing sooner or later...
it was important to me, that i stick to the game and play it, so i was sure to get the HOTAS first because without a HOTAS in my opinion the game is not playable with fun (I tried it with my old MS Sidewinder Force Feedback II) so, get the HOTAS first... then see if you like the game and stick to it... soon after you're in the basics and able to fly, you'll find out that you need to use buttons to change your view, and you'll find out that you need your eyes to spot a SAM launch etc. so you'll get the TrackIR :)
in short words, get the HOTAS first... its much better to learn the controls on a HOTAS and then, after you're good with it, you'll see for yourself the things that make a TrackIR necessary... if you do it the other way, you'll have to change the whole controls...