Strike Squad
Basically these are space marines with mastery level 1, the ability to Deep Strike and all start with an AP3 Force sword. They are 20 points per model and actually for the cost are not that bad. Their problem is that they are not Relentless and so can't utilise the special weapons effectively but more than that they are only 5 points cheaper than Purifiers which are 23 times better plus 7th doesn't care about Troops as much. Also terminators are a better troops choice…
Terminator Squad
These are only 13 points more per model than Strike Squad models and are twice as durable against small arms fire and infinitely more durable against AP3 or better weapons when not in cover. Plus they have 2 attacks rather than the 1 that Strike Squad guys have and of course they are Relentless so can move and fire the Psilencer and Psycannon to full effect and then assault. It is also worth remembering that Grey Knight terminators carry both frag and krak grenades unlike regular space marine termies and so can still strike at full initiative with their Nemesis weapons when charging through terrain. In fact I would say that their only downside when compared to a Strike Squad is the fact that they cost more and the special weapons also cost more (by 5 points each), but also they can't sweeping advance which is a fairly big deal sometimes, but in no way off sets all the positives.
In conclusion don’t bother with strikes, load up on termies (by that I mean fill your 1 minimum troop selection with a 5 man terminator squad!)
As Matt says, in 7th edition Troops aren't as relevant as they once were. In the Wolves book you'd probably want some of them anyway but here I think there are better units to spend the points on. Of course, this assumes you're using the GK-specific detachment so Objective Secured isn't an issue.
The next post will cover the Elites section of the book and I have a fair idea which unit will shine there!