SSD's have no moving parts, can be used extreme conditions, preferred by the military for use, have a lower power consumption, So I would think they are far Superior in reliability.
You can do a search and find a lot of information on comparisons about reliability the SSD wins every time.
A quote for Storage.com on SSD's Vs Hard drives Reliability
In terms of reliability, conventional HDDs pale when compared to SSDs. The absence of mechanical arms and spinning platters is the reason behind its reliability
. In demanding environments, SSDs provide the type of ruggedness required for mobile applications. Unlike the HDD, SSD's can withstand extreme shock and vibration with data integrity and without any danger of data loss. This feature is very important in industrial applications where exposure to highly combustible materials and electromagnetic radiation are typical. Their ability to deliver unnerving performance in extreme conditions also makes SSD play a vital role in military operations, be it in defense, aerospace or aviation applications. Military
applications require, in most cases, an operating temperature range of -60°C to +95°C. Shock, vibration, and temperature ratings of HDDs cannot comply with military standards, only SSDs can.