I prefer to use LitteNavMap to generate IFR flight plans, however with the new way of having to file flight plans (not directly using vPilot), I’ve begun using Simbrief. I do notice that the two flight planners often generate completely different routes for the same flight. Is one planner preferred by the controllers over the other? Is one of them generating incorrect flight plans?
Many countries have standard routes for city pairs. Simbrief knows about many of them. In fact some VATSIM divisions provide these to Navigraph so it is more likely to generate a valid flight plan.
I mostly use Simbrief for IFR planning and LittleNavMap for VFR. Occasionally I find that Simbrief does not recognise an out-of-the-way airport code but LittleNavMap does.
With Simbrief the first routing offer is not always the most direct nor the most efficient if you are flying ‘real-time weather’. There’s a list of routes available or, of course, you can create your own route.
You probably know all of this so please forgive me if l am spouting the obvious.
Depending on what you have LNM use as a database, you have to be careful with these “out-of-the-way” airports and their ICAO codes. For example, I use the MSFS library for the airport/scenery data. The MSFS library has a code for every single airport, so if an airport has no official code IRL, LNM will just show the fictitious code from the sim library instead. To controllers and other pilots, that code will be entirely useless because it doesn’t actually exist outside of the sim library. The correct way to file to/from them would be ZZZZ as the destination in item 16/origin in item 13 and DEST/ or DEP/ in item 18.
With airports that do have an official code, I have found that SimBrief usually has them. The only exception to that seem to be heliports.
Here it’s important to note that SimBrief often has route suggestions with a “Real World”-, “Eurocontrol”-, “NATS”-, or similar icon. On those routes, you can be reasonably sure of their validity, but particularly in busy and complex airspaces like central Europe, that means slightly less efficient and less direct routings because more direct routings would cause all sorts of problems, so if you don’t want to go through the tedious process of finding something valid on your own, SimBrief often helps you avoid potentially lengthy delays at your departure airport due to an invalid flight plan. For most airport pairs that the average VATSIM user would normally fly between, these validated recommendations exist - if you prefer smaller airports or more exotic airport pairs, it becomes a bit trickier though (but even then you can often build something reasonable from SimBrief suggestions for nearby airport pairs).
LNM, to my knowledge, has no such tools, which means that you are much more likely to end up with some egregiously invalid routing.
This all applies to IFR, of course. With VFR, it’s usually much easier to create a valid flight plan because VFR flight planning rules are much less involved.
In that regard, I would also say: when flying IFR, SimBrief makes things much easier and faster than anything LNM could provide. For VFR, on the other hand, LNM is a godsend, especially if there is a map plugin with an official or at least highly accurate localized VFR enroute chart. Oh, and whenever I feel like doing some old school IFR VOR-to-VOR (or maybe even celnav once I learned that) in the DC6, I also plan that with LNM because SimBrief is very much focused around RNAV-capable aircraft/flights.