i was in the same situation last year, with 08 4.8i almost fully loaded too, with approx same $ for buyout. I did the math of what it would cost me to get a new 3.5i, 3.5d, 5.0i or keep existing and trade it in later - over the period of following 3 years (lease term). All things equal (meaning I'd buy extended warranty and maintenance programs to receive same "ownership experience"), this is what I came up with ($-wise):

keep mine vs. lease 5.0i -> keep mine, as lease would cost for 5.0 ~6K more over 3 years
keep mine vs. lease 3.5i -> comes out even $-wise, thus makes sense to lease and drive new car with new features
keep mine vs. lease 3.5d -> lease 3.5d, savings of ~4K (incl savings on fuel)


So it all depends what you want to get going forward... If you want crazy acceleration 4.8 provides, you must go with 5.0i, therefore keep yours. However, if you are willing to compromise a bit off-the-start kick, go with 3.5d and lease yourself 2011!

EDIT: Another important factors are, obviously, your state of finances, savings, and security of your job/business. If you end up paying off your 08 in cash, make sure you don't significantly deplete your savings by doing so. In my case I did a compromise - I put some money down in the begining to keep monthly payments around $500. So instead of spending $43K (taxes incl) to payoff my lease, I put down ~ $14K and have relatively low payments. So, g.d forbid we end up loosing jobs, we still have good savings and relatively low monthly payments to get by.

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His : 2013 X5 3.5d Deep Sea Blue/Black/Aluminum with ALL packages, 20" 214"s, running boards, Adaptive Drive, Spare Tire, BMW Apps
On Order : 2016 X3 3.5 MSport Silver/Mocha with DHP, CW, Prem, Tech, DA, Lighting

Hers : 2015 328xi MSport Silver/Red

Prior:
2008 X5 4.8
2011 X5 3.5d
2010 335 xDrive
2013 335 xDrive