I started with DCS and then added Legacy several years later. Overall IMO Legacy is easier to set up and manage. Here are the problems with both systems:
DCS:
Requires a more complex star pattern in wiring, older TIU versions (Pre-"L") produce weaker signal
TIU "L" versions have stronger signal but are susceptible to damage to signal components (unless circuit board modifications are done)
Neither TIU or Wifi-TIU are currently available
PS2 5V boards in engines have a higher failure rate (3V boards are more reliable), PS2 boards don't receive signal as reliably as newer PS3 boards
No handheld remote option with new equipment (phone app only)
Fewer new locomotive models being produced
Availability of boards for engine repair or upgrades is spotty
Legacy/TMCC:
Signal problems can occur with parallel tracks, bridges, tunnels, or if house wiring does not provide adequate earth ground (solved by adding a ground plane wire in problem areas)
Many different configurations of TMCC and Legacy engine electronics make DIY repairs more challenging
Lionel abruptly stopped selling all replacement electronics boards for TMCC and early Legacy engines. Future availability is unknown. Electric Railroad Company continues to supply boards for TMCC upgrades and repairs (sound sets are more limited and generic)
Legacy remote and Cab2 base no longer sold, Cab3 base not available yet
Only new handheld remote option will be Cab L which has no display screen (phone app is available)
Conclusions:
Most of these problems can be solved with time, money, and patience. You may want to wait for the new control systems to be available. You can purchase a used DCS TIU and remote ($500-700) or Legacy Cab2 base and remote ($800-1400) on the secondary market at greatly inflated prices. Beware that most TIUs being sold are not Rev "L". Most auction site listings "conveniently" forget to list the version and don't include a photo showing the back of the unit or a view of the ports (where version is could be identified).
Eventually, new control systems for Legacy and DCS will be available. But many of us prefer the handheld DCS and Legacy Cab 2 remotes, only available as used equipment.
As I stated, I have had fewer signal problems with my Legacy system, but I have an earlier version of the TIU and my MTH engines are all PS2. I run both since certain engines have only been available from MTH (DCS). If you go with DCS, sticking with a Rev "L" TIU or WTIU and PS3 engines will be most reliable.
Bob