[MENTION=71973]bluecollarjock[/MENTION]
I like the WK2 grand cherokee, but if you spend enough time on the jeep forums look at how many common problems and complaints there are. Even the people who said "yeah, definitely get the WK2 over the 5th gen" were posting threads like "Back in the shop... again" for all kinds of issues. The WK2 has an awesome interior, and all-independent suspension makes the road handling nice, but the reliability issue is a giant hurdle to overcome.
You mention fuel efficiency and power, but the only grand cherokees that arguably have both are the diese*l options which are more expensive. For the gas engines your options are either slightly better fuel efficiency, or worse fuel efficiency and more power, but not both. (*and don't forget that you can't compare mpg to mpg for diesel because the fuel price is different, you need to convert to $/mile)
You say that it will come down to what kind of deal you can get, but you need to consider your resale as well. You will be able to find better deals on WK2's, but that's only because 4Runners hold their value so well and don't have the same amount of depreciation. The WK2 may be less expensive now, but when you go to sell your vehicle you will also get a lot less than you would if you were selling a 4Runner so you need to look at the total lifetime cost of ownership (and when you include maintenance costs in that, the WK2 can get pretty pricey).
I was in your shoes last year, I really wanted the Grand Cherokee (it looks awesome and the interior is awesome) but the 4runner was just the no-brainer, easy decision, much more reliable vehicle. If you want to spend $20k-$30k on a nice but unreliable vehicle you should also consider used Range Rovers.
Take a look at these:
Transmission problems:
2015 Jeep Grand Cherokee Transmission problems - Please help! - JeepForum.com
Quadra Lift Problems:
Quadra Lift Problems =( - JeepForum.com
Vehicle dead won't start :
2014 gc dead won't start - JeepForum.com
Wiper motors failing:
Rear Wiper Motor Failed...anyone know how to replace? - JeepForum.com
dimming mirrors not working:
Drivers side auto-dim mirror not working - JeepForum.com
people having headlight problems not covered under warranty:
HID Headlmp Assembly NOT covered by Maxcare!! - JeepForum.com
water pumps failing early:
Well this sucks - JeepForum.com
blend door gears strip:
No heat on passenger side - blend door gears - JeepForum.com
more vehicles dying on the side of the road:
My 2014 Jeep GC Diesel Summit broke down today (bad) - JeepForum.com
steering wheel buttons stopped working:
Srevice Airbag/steering wheel controls don't work - JeepForum.com
more quadra lift problems:
Srevice Airbag/steering wheel controls don't work - JeepForum.com
and on and on and on.....
If I spend $30k and have to put up with all of that, I would rather just buy one of these for the same amount of money: