I don't find any GPS very useful for trip planning. They are all too focused on a point to point trip. It is very hard to customize routes to avoid certain things, to work out multiple stops, etc.
You cannot really work out multi-day trips easily.
S&T as long as you have the install disc will be usable for a long, long time. You can plan trips with S&T, and use a GPS for a daily route.
Modern GPS units do have a lot of customization possible, so after you get one - run a 30 mile or so route several times trying different options in the way it routes, displays information, etc. Learn how to use it well.
Finally two points - when towing/ traveling in an RV - NEVER use the 'Shortest Route' option. That will send you down the most unsuitable roads just to save a couple hundred feet.
Secondly, never trust a GPS to be 100% accurate. Know where you are going, know which major roads you are going to take, and ignore the GPS is the directions seem inappropriate.
Review the entire route with the GPS before starting the journey. Almost all GPS unit have the ability to show you how they have the entire trip planned.
When we tow for a day, I work out a route sheet for the wife to cross check our progress against a map. I use S&T to tweak, plan that route.
I have two GPS units - a RandMcNally Good Sam RV GPS which is very easy to use and warns me about weight restrictions, height problems (I set it to 14 ft clearance, even though my rig is under 11 ft), steep grades, etc.
I also have an older Garmin with Lifetime Maps & Traffic - which warns me about current traffic delays. Sometimes when I have to go off route due to traffic - it means stopping in a big box parking lot, pulling out the computer and replanning the route. But that beats sitting in long traffic delays.