For solo play, there is no good reason to stay on just 2.5
2.7 is
Faster AI and Turn processing than 2.5
Requires less memory than 2.5 (great is you only have 4-8GB of RAM)
Access to some good looking 2.7 maps, that use
isAI to aid the AIs game play
Enhanced Purchase Panel: It is improved to remember and retains your older troop purchases when you accidentally overbuy.
Shows on Status Bar, Territory Units: A quick-glance now lists the exact units in a given territory for better tactical awareness.
Shows on Status Bar, the TT/SZ relationship(war, neutral, allied) of territories (Really good in FFA maps)
Shows on Status Bar, Map Zoom: Map viewing is more flexible allows for a custom zoom capability up to 200%.
Shows any map missing icons as a placeholder square icon. (A must for map makers)
Get it here
https://github.com/triplea-game/triplea/releases
You can also have 2.5 installed at the same time.
Written for 2.6, but it still applies to 2.7
https://forums.triplea-game.org/topic/3724/how-to-have-triplea-2-5-2-6-installed-at-the-same-time