It all depends on what you are going to use it for... I mainly post in the beach section and my detector of choice is the TDI pro and a TDI beach hunter that I already modded.
If you are looking for depth, with a couple mods few machines can touch it... On the SL you do need a slower swing speed than the TDI Pro.
In a way it is a niche machine, but once you understand what it can do this thing is a monster. Most that have things to say about it, never used it or only used it for a couple hours.
The SL has a smoother threshold compared to the TDI or the TDI Pro... So really the thing is what are you going to use it for and what can you get it for...
Any questions holler...