Is there an obvious scientific reason why one method is better than the other? Or is there an “experience and taste” reason, even if not quantifiably scientific, that would make one way better?
No, one way is not better than another. I was just thinking that since you are not familiar with what the final beer will taste like, leaving the lactose out would be prudent. That way you could taste the final beer and decide how much you wanted to add, it might not need any at all if it finishes sweet. Or you might only want to add half as much. Not that lactose is really all that sweet, but I’m a fan of waiting and tasting before deciding what to do.
But since you already added it, RDWHAHB. 
Don’t be afraid of pitching an attentuative yeast for a milk stout. (5 gallon batch) I added 1 lb of lactose ( at 10 mins left) , and pitched SafBrew S-33, highly attentuative and still ended at 1.030. A nice sweet, milk stout. I would actually be careful about a Whitbread yeast, possibly leaving it too high, maybe? Pitch a yeast with good attentuation, don’t let the OG fool you. It’ll still finish high, the lactose is not devoured by the yeast.
So i finally had time to brew today
here is what i ended up with
1lb black patent malt
3/4lb chocolate malt
1/2lb carapils
1/2lb crystal 80 (English)
1oz cascade 60min
.5oz styrian goldings 60min
.5oz styrian goldings 30min
1lb lactose 13min
WLP002- English Ale Yeast
6lbs light LME
3.5gal boil
waiting on the wort to chill (haven’t purchased a wort chiller yet) can’t wait to taste it once it cools down
Looks tasty. My wort was pretty darn tasty when I tasted the gravity sample. Quite bitter (normal for wort, so I read), quite sweet, and definitely promising! Make sure to let us know how it comes out, I will too!
I just got my wort chiller. Need to get a different fitting for the sink, it came with one for an outdoor hose, but that’s no big deal. I won’t be brewing for at least another week or two anyway. I don’t even have any ingredients for my next brew yet!
My tasted pretty darn good, little bitter, but sweet, some coffee and chocolate notes should be better once it’s carbonated. however, i was letting my 5yr old cousin help me brew and he added the yeast before i took a OG reading, so i guess i will be using the estimated OG.
you can still take a gravity reading after adding the yeast. as long as it hasn’t started fermenting which takes a couple hours at least.