5th Suicune or 1st Shedinja?

There really isn't a right answer here. Barring some massive rework, Suicune unfortunately is no good in Pokemon Go (even when it had Hidden Power it was still bad). That said, unless Shedinja ends up getting Wonder Guard, which is unlikely, it's just going to be a Dex entry.

Neither will really be worth investing in, so what it comes down to is if you want another shot at a trophy hundo Suicune or to be amongst the first with a Shedinja.

You also need to consider the cost of foregoing research for a day or two, including the rare candy task (evolve 3 shuppets/duskulls).

5th
+ it´s a legendary
+ not so many chances to trade it for a lucky one if needed
+ shedinja is not legendary and i think you´ll get it later on other ways
- you´ll get shedinja later then other players
+/-both are useless

Potentially you can use the candy from shedinja to evolve a nincada later when it is released. But I bet ninjask is just as useless