We had just over 3 WPG on a 125 (384 watts) and the softies we had did fine. However, the brand seemed to make a difference. on the 125 we used a custom sealife(now out of business, but someone bought them out). It worked terrific. We then purchased another brand (al cheap O)for a 55, that was over 4 WPG (220 watts) and or corals started to die and look horrid. The custom sealife we had, had the moonlite built in and it was a great setup. So i would recommend not going the cheap way on the lights. Get a well known brand.
you could probably get away with 2 48" coralife aqualights which hold 4 x 65watt bulbs each for a total of 520 watts or 5.78 watts per gallon(I agree watts per gallon is not a good basis for judgement). You could keep soft and some lps alive but very few things will thrive. The problem is the depth of the 90 gal which is around 24". if you ever want to see any growth on your corals or keep anything other than soft corals alive, you will most likely need to upgrade to Halides. If your tank was shorter, (20"-21" like a 75 gallon) you could be more confident in the selection of PC. I personally would go with 150w or 175w halides with PC or VHO for actinic.