Here's my 2 pence, but bear in mind I am not massively knowledgeable on this kind of subject, however I have had Sportlines, KTEC coilovers, and now KTEC coilovers with a rear adjustable ARB (similar to the Whiteline one).
Sportlines = Great handling, comfortable, although were a bit uncomfortable near the end as I think my schocks were going. Sat a bit high for my liking
KTEC coilovers = Ride height is spot on now (obviously, as it;s adjustable), much firmer ride which is great on good roads, but over bad country roads it's a little uncomfortable and can be a bit lively. Handling wise, I never had much confidence, it felt like my car understeering a bit too much, but obviously this could be down to how they were setup (although I did have laser alignment and stuff done, but ride height was done by me)
KTEC coilovers & ARB = In my opinion this is the perfect balance of looks and handling. The car handles better than when I just had Sportlines, it's incredible (even though my tyres need replacing). I can't wait until I get near tyres later this week. It is just point and go, no understeer, no uncontrollable oversteer. I can fling it round corners now and pretty much put it where I want it. Comfort is still a slight issue, it's a little bumpy and when I have passengers in the car (rarely) I do feel a bit awkward about it, but on my own I don't care too much.
So, my thoughts. Sportlines and ARB I have no doubt will be incredible, and if you want a comfortable ride and great handling, without too much concern for it looking low then go for it. Sportlines obviously lower it a bit, but my personal preference was to have it a little lower than that. If you're not fussed about ride height, it's a no brainer; go for Sportlines and ARB as I'm sure it will be amazing.
If you want it lower and don't mind a slightly bumpier ride, then go for KTEC coilovers and if you can stretch for an ARB now, or further down the line, then I'd highly recommend it. I'm not sure I would recommend KTEC coilovers on their own though, as for me before I fitted the ARB the handling was the same, if not slightly worse than standard, although the ride was firmer and there was less body roll.
Long and short of it, if nothing else, get an ARB
I hope this helps, these are of course just my opinions based on driving with standard cup suspension, sportlines, KTEC coilovers, and now KTEC coilovers with an ARB. I am not an expert.
To summarise, based on my experiences, handling performance (on good roads) was from best to worse:
KTEC coilovers & ARB
Sportlines
Cup suspension
KTEC coilovers
I imagine Sportlines & ARB would be top of that list if I tried it.
