Choose XT where:
A) there is additional functionality or features over SLX that you will use (be relly honest with yourself)
B) XT looks a LOT better and it’s a big item that you will really notice and regret not spending a bit more for XT
Choose SLX where:
A) there is no additional features and/or significant weight reduction for a fair whack less.
B) for something that is highly likely to get damaged.
Personally I would go 10 speed with a double up front. It depends on the ratios as to whether you go for the XT double or the old 9 speed SLX double. Of course you could also go for the XT double and sell the rings and then buy the SLX rings. For the rest of it I would definitely get an SLX FD, possibly RD and maybe opt for XT shifters if they are still nicer like the old 9 speed ones as you actually directly interact with the shifters so will notice the difference. Or on the other hand if you don’t care about a bit nicer feel then get SLX shifters as they will work just as well.
For brakes I am not sure what the extra functionality of XT is but unlike the older brakes the SLX is right up there being almost identical design.
Bar – Easton is my first choice but there is lots of good bars out there, especially if you want wide aluminium.
Stem – Easton haven is lovely, Thompson is the default, RF do make some nice ones (I am not a fan of RF cranks but the bars and stems are very good value and well made). Nothing else interests me unless looking for sub £30.