In my opinion all classes require from @ endgame to know the fight itself, as well as knowing your class capabilities and aspects.
If you do not know the fight mechanics, you will not have optimal gameplay with whichever class you are on.

Each class has its own difficulties and nuances. There is a lot to handle for basically everybody at end game. At 50lvl you have plenty skills, buffs, off-GCD abilities and in all really counting as an endgame fights, you have other things to take care off - mechanics of an encounter.
I would suggest to try out different classes and figure out which style you like most. Eventually you want to level most (if not all classes) to certain level for cross-class abilities.

Regarding BLM and SMN.
This comes down basically to personal playstyle preferences. This game has great balance in classes so whichever you pick, you will do as good as comfortable you feel with the playstyle for your job.
Personally, I do not enjoy BLM playstyle at all. I've been trying to force myself to lvl it up to 50 as it would share everything but the weapon with my Summoner. However, lvling this class is so damn boring to me that I postpone it and choose to level other classes whenever I feel I could use some break from my Summoner

Now, before going forward, I would like to mention that I do not have BLM lvled to 50, so my opinion regarding difficulty is based just on my observations, opinions from my buddies and forums.
I will not take into consideration the aspect of moving during the fight as it affects more or less all the classes. Although, BLMs dps is dropping most from what I have seen due to movement.
Playstyle of BLM seems pretty straight forward to me as other than:
- watch for your procs to get activated
- and anticipate incoming AOE,
you do not really have much to take care off.

Summoner is considered as a difficult class from what I hear ingame and on forums.
I feel quite comfortbale with this class and haven't found it difficult much. Of course learning all the time how I can maximise my output in particular encounters and so on, and so on.
Summoner has most likely more things to keep an eye on:
- micro managing pet - positioning, using pet's attacks when appropriate
- watching out for dots and refreshing when appropriate
- managing your Aether stacks
- rezzing people (you should be the first one in line to rezz fallen comrades)

At the beginning it felt a bit overwhelming in difficult encounters when I was not that familiar with the class, yet.
However, after a while I would say that all classes require just muscle memory, knowing the encounter, understanding your class abilities and just common sense.

Long story short - try out both and see which suits you more. At around 35 lvl you should have already a feeling of how the class will be played at endgame as basically playstyle won't really change at endgame, will just get adjusted to new abilities