In short the Td5 is a little underpowered for the size and weight of a DiscoveryII. All in all a nice engine with enough power when you need it. I've had three of them and never had any power issues in rocky or sand terrains.
The V8 is a something totally different. It also has heaps of torque, much more power and in general I'd almost say a nicer drive because it is more reactive. The downside is the fuel consumption and therefor the amount of fuel you have to carry along on long trips, but in the long run the cost of ownership would probably be not far apart between the two.
In essence both are good and purely boils down to the question - do you liek petrol or diesel. Get whatever one fancies you.
2012 LR Discovery 4 SDV6 SE
2011 LR Defender 90
2014 Jurgens Xplorer
Disclaimer - Everything I post is my opinion, based on my assumption or experience, might not be factual, don't have to be justified and might differ from other opinions. Deal with it.