I was recently doing a similar calculus.  I just bought a BK-X (arrived yesterday in fact).  Basically because it has more options to mine, and they can add algorithms.  When they released this as the X10 it only mined 5 algorithms and they added two more.  It seems likely that they will mine for themselves initially when they build the miner software to mine more algos but they did in fact eventually release the firmware to mine the other two to everyone, so it's certainly possible that they could continue to add more later.

The decred miner is less versatile and in the longer term that means it will probably have a shorter useful lifespan.

I don't have the money to buy all the miners that come out, but I like to diversify a bit.  I think the BK-X is a good choice for something that is likely to make money over the long term but some of the single algorithm miners can make really good money if you get in on an early batch.  But the way things are going, single algo miners are a bit of a gamble.  The recent X11 miners burned everyone who bought one.  But some of the various blake algo miners made insanely good profit for the first people who got their hands on them.