From someone who used to use both behind the keyboard, HPtuners all the way.
From an end user argument that diablo is more user friendly, I argue, you need a computer to save the diablo tune to, then upload to the handheld then upload to the vehicle, then log, then upload log to computer and email to tuner. Repeat for subsequent revisions. With hptuners you realistically need a laptop, but of all the adults I know I cannot think of many that do not have, nor can afford a cheap laptop. With hptuners, open an email, open file, upload. Open scanner, log. Save log and email to tuner.
In the end, a custom tune is only as good as the person behind the keyboard making the changes. However, that being said, there are some things that are just not possible with diablo. One of my biggest complaints is there is no way to log VE in order to properly tune for a cam. You can take some fuel out of or put some in certain areas of the VE table and blend to get fairly close, but there is no way to log a correction chart to make proper changes.
The parameters for transmission tuning are much more complex in HPT allowing for a better overall transmission tune if you want much more than just line pressure and rpm changes.
Another great benefit of HPT (imo at least) is the end user can make minor (or major) changes if desired. With diablo you must first purchase CMR software which is fairly expensive (iirc I paid $1500 back in 2017) Hptuners might be slightly more expensive for an 8 speed vehicle between the MPVI2(2+) and credits to license the trans, but it can be used for as many vehicles as you're willing to purchase credits for
In the end, the best advice I can give, is decide who you want writing the tune(s), discuss with them. Even when I was tuning with diablo also, there were certain builds I did not like doing with diablo. Eventually myself (as well as several other tuners I've noticed) have gotten away from using a software we don't care for in order to provide a better service to my customers.
The person behind the keyboard is the number one thing affecting the quality of the tune. So defiantly see what they are more comfortable with and go that route