I don't have any specific knowledge about this product, but I would guess this is an example of binning. They're not really different, it's just a matter of how fast those particular units are able to go.
Most likely, both kinds of units are produced with identical parts and processes on the same assembly line. Due to normal variation, some units will be more robust than others, so they test each unit at various clock speeds to see how fast it can successfully operate. The 14 TH/s units passed their tests at a higher clock rate, so they set them to operate at that rate and sell them for a higher price. The 13.5 TH/s probably failed at the higher rate but passed at a lower rate, so they set them to run at a lower rate, advertise them as meeting a lower performance standard, and sell them for cheaper.
So in that sense the 14 TH/s units are probably "better" and perhaps more reliable overall, but then again they cost more. You'll have to look for product reviews to try to estimate how reliably these units perform as advertised; that's beyond the scope of this site.