Both are excellent choices.
Having upgraded from the DAC2 to the DS ( kit form ) and already powering my digital front end with a P300, I can say that the difference was night and day in my system.
I have a P300 and a P3 in operation and I would not be without them, but the DS drove home for me the importance of having the best source possible as a anchor to build a system around.
If you decide to go with the DS upgrade you have a few options to consider;
Sell your DAC2 ( they sell for around $1000 ) and buy a second hand DS ( they sell for around $3000 - $3500 )
Or trade in your DAC2 ( PSA is offering $1980 on a trade in ) and the DS will cost $4019 delivered, not bad for a brand new DS.
Or by the DS upgrade kit for $2999 ( maybe less ) while they are still being offered.
The only difference between an upgraded DAC2 and a new DS is the IR receiver, the new DS has better off sides reception. ( a round receiver vs a narrow slot receiver )
While the P5 family of power plants are great, you may want to consider holding out for one of the new generation power plants, maybe the yet to be released new P300, if your load permits.
You can probably even trade your BPT in for a credit towards a purchase of a new power plant.
Also keep in mind that any new PSA equipment will have a 3 year warranty, even the DS kit.