Alrighty then, I'll try to be brief. I'll even use pararaphs.
With a forward pack in general, you need to look at what you've got available and how it is going to play to your strengths across the field.
Tight five: win set piece first and foremost, contribute in the tight-loose.
A lot of teams have leaned toward a big bopper TH lock to stabilise the scrum, and a skinnier aerial target on the left side e.g. the Boks with Botha and Matfield. If you can get two big units who are also good in the air, even better. Similarly in your front row you want to know their scrum work is up to scratch but then look at what they bring around the park - are you just looking at each prop to get to every second ruck? Is the hooker going to be a link man or a fourth backrower style? There are several questions alone in the tight five, but if you're not winning your first-phase ball, your back row balance can quickly become irrelevant.
Back row: set defensive tone, own the ball, provide link to the backs
Back row needs to take any extras from the tight five into its mix, but should also look to offer something to the set piece. Additionally, your back row need to operate in a complementary fashion with each other. In the old days you could have two fast flankers to hassle the opposition at ruck time because the lineout was a bunfight anyway, and a big unit at 8 who was your primary ball carrier and link man.
These days it is a little different, but the starting point is that they all must be in the top few defenders in your team. Typically your back row now is still a big ball runner at 8, with a lineout target blindside and someone good on the deck at openside. If they bring more than that, great!
This conversation goes to Hooper; he's the captain, he's going to be first picked, and he's rarely going to be on the sideline for the 80 minutes. So let's accept that he brings us ruck work (both sides), speed on attack, and link play. He's not a crashball guy tho he's got the balance to shrug tacklers, but won't run flat out at a brick wall. So that largely fits the bill for 7. He's not as good over the ruck as someone like Pocock, but then Pocock wasn't ever a great ball runner.
From here, you can pick your 6 first or your 8 first, but you've got to be able to fill the gap with whoever comes second.
Let's start with 8, and consider Naisarani, Valetini, and Wilson.
The first 2 are similar - big units, crashball at and over the Ad-line. Can be a lineout target but really you want to put in a 5-man and have them as a running target out in midfield. While they can contribute at ruck time, on offense you want them running at the opposition with ball in hand, because that's their job.
Wilson can run the hard lines but his strength for me is more the linking role with good hand- and foot-speed through the line. Good lineout target as well, as he's a bit more agile, and IMHO edges Isi and Rob at ruck time as he's got more fitness and speed. For me, he's that link role that Kieran Read did so well with McCaw and Kaino at 7 and 6. You lose a bit in hard running, but gain in speed and hands.
So what gaps does the blindside need to fill in both these cases?
Regardless, we need a genuine third lineout target. Ideally someone the height and frame of Fardy, who can operate at the front as the safety ball or the trick play at 6-8.
If your 7 is going to hunt the ball and your 8 is going to crash it up, then the blindside also needs to get in and do the dirty work with the tight five at times, as Hooper can't get to every ruck. Additionally they need the pace and tackle accuracy to protect the blindside off the scrum. Some days that blindside will be notable for his involvements, but some days you might not hear his name at all, and only see some good stats after the fact.
Onto specifics: if you pick Valetini and Naisarani you're picking two very similar players. They're both big ball runners. They're both capable lineout targets, tho again you'd want them out in midfield for the crash. For me they occupy too much of the same space, and as a pair they don't complement a back row with Hooper at 7.
Valetini plays 6 at the Brumbies, but he's playing more of a crashball 8 role there, when Samu and Brown are his partners. Balance.
Naisarani was playing more of a pure 8 role at the Rebels with openside of Hardwick and a blindside like Kemeny. Balance.
Therefore, in my humble opinion, if you pick Isi or Rob at 8 - and either is very capable - then you can't have the other at 6.
Does that automatically make Swinton first-choice blindside? Maybe not, but then you'll need to find someone better. Wilson might add that string to his bow to ensure his longevity at this level, but probably not next week - let him have a crack against Argentina.
