Aside from the cost (piston setups are noticeably more expensive), you'll probably get the same level of reliability and durability with either a DI or piston system.
From a cleaning and maintenance standpoint, the difference between a DI and piston system is night and day. The piston system runs far cleaner, cooler and will probably experience less wear on certain key components (BCG).
You honestly can't go wrong with either a DI or piston, but if it's in your budget and you have less time for cleaning, maintenance, etc. - a piston probably fits the criteria better. With respect to the comments about carrier tilt, this isn't something you have to be concerned with on any of the modern setups.
I would also echo scott585's comments that if you want a piston system get a dedicated, purpose-built one (and not an add-on or conversion kit). Core15 is on the lower end of the scale, with PWS, LMT and Adcor in the middle and LWRC and HK having the best piston systems (also $$).
Good luck!
&
