Parti-Gyle help

I'm planning to brew a Kate the Great Stout clone that Jimbus posted a recipe for a few years ago and the thread is still going and quite long. I've made my way through all of it and plan to brew this beast tomorrow. I've also read about Parti-Gyle brewing and plan to experiment making a second wort with the same grain since the first is a high gravity 1.104 OG Imperial Stout. I'm debating between a chocolate stout or a black ipa for my second wort knowing it will be much lower gravity.
I have not yet tried parti-gyle brewing and welcome any advice or knowledge from more experienced brewers. What would your choice be for the second runnings from a strong stout like this? Choc stout, black ipa, porter, something else? Is parti-gyle worth it? Would you add grain to the second mash for more color, flavor & higher gravity?
I have an extra yeast pack that I'd like to use: Safale us-05
Thanks for the ideas and advice.

Definitely take gravity readings the whole way so you know what you're getting from the second wort. I bought a refractometer just so I could feel confident on the second batch. Maybe not practical for you, but numbers are a huge help in parti gyle. I ended up adding 2 lbs to my last parti gyle batch and for the extra ~$4 i say its more than worth it to get an extra fermenter full of wort when i would normally get only one. Edit: i added them as steeping grains in the boil. These websites are also helpful in the planning stages:

http://morebeer.com/brewingtechniques/library/backissues/issue2.2/moshertable.html#

http://www.brewersfriend.com/ibu-calculator/

http://www.brewersfriend.com/dilution-and-boiloff-gravity-calculator/

Here is an important question: do you plan on mashing the dark grains with everything else?

Dark grains, like crystal, don't really need to be mashed. So you have a number of options for treating them. Depending on the recipe, you could cold steep them separately from everything else (overnight at room temperature should do the trick), then pour that liquid into the boil with, say, 10 minutes to go. That will yield a very smooth, less harsh taste, though you might not want that with an RIS.

Even if you don't do that, though, you could hold back the dark grains for a separate steep (say, in a stockpot). This would allow you to use the "base" malts (or whatever you put in the mash) to make first runnings for an RIS, and second runnings for just about anything. I don't know your recipe, but you can imagine examples--an 80% pale, 20% Munich base could be used (when dark grains are added) for a stout, and then the second runnings could be used for an IPA (with different specialty grains).

If you do mash everything at once, you're a lot more constrained. Bear in mind that the second runnings will have less desirable flavor--they may be somewhat more grainy, less malty, perhaps harsher tasting. That might counsel against making a straightforward malty beer (e.g. brown, porter) and in favor of making something that relies on other, more complex flavors, like hops or fruit, which would mask the roast. When I recently made an RIS, I used the second runnings to make a cherry stout (still aging), on this theory.

I just wrote a great response thanking you guys for the advice and help, then my phone froze and it erased. I went with a black ipa for the second brew to hide any imperfections from the grain mashing so long and I did add some dme to my second boil to get an appropriate gravity reading. I took gravity reading pre and post boil as wellI I mashed all grains to follow the RIS recipe fully. Your advice is much appreciated and I have a better understanding for next time as well as the links you provided for more reading to do to prepare properly for another long brew day like this. Both beers are bubbling away in the basement now. Fingers crossed they turn out as good as I hope they did.