Replace SPD pedals for road pedals... which system to choose? I ride mostly two bikes: one for commuting/touring, one for training (and soon racing).

I ride SPD on the commuter and installed SPDs on the racing bike that I bought a bit later so I did not have to buy new shoes just for this.

Problem is that as Iam taking training/racing more seriously, I would not want to be limited in the choice of my shoes when I get to buy some in the future, and the only SPD compatible ones are either touring/commuting/MTB or very bottom-of-the-range road ones that have both the 3 road drillings and the 2 SPD drilling.
I want to be able to choose some shoes that have very road specific features such as lower weight and very stiff soles...

Plus, SPDs are fine for touring/commuting/MTB but if they're not commonly used for road training/racing, there has to be a reason...

So this all leads me towards a switch to road pedals...

Just wondering about the system to pick:
- standard Shimano SPD-SL that seems to be extremely standard, good value (both the pedals and spare cleats)
- Time? Look? Mavic? that seem to their fans
- Speedplay: I really like the 2 sided system and there seem to be good reviews but they're expensive, bot pedals and spares...

What should I pick?

Consider that I will continue touring and commuting with SPDs so, while I am ready to lose shoe compatibility on all bike, I don't want the transitions between systems to be too tough, hence my liking of the Speedplay system and its double sides ...
I've ridden commuter and road racing bikes both for decades and can validate your decision to go specific in regards to your shoe/pedal setup.  Start with the shoes - try a bunch on and find a pair that really fit well and seem to work for you.  Most modern shoe/pedal systems use the three bolt pattern common to Shimano and Look systems so there's not a lot of compatibility issues anymore.

Speedplay is the outlier for sure here - I wouldn't worry so much about single sided vs double sided.  Getting in and out is a bit trickier for road systems but you'll figure it out quickly regardless.  What you want to concentrate on is float and release pressure.  Since you're used to clipless on your commuter, it will be fairly straightforward to try systems out.  Work with a retailer who will let you try different systems while using a trainer, that way you can get a feeling for how they work.  If you don't have a retailer close by who will let you do this, look for a fitter and ask them to fit you.  It costs about $100-$150 to get fitted properly with new shoes and pedals on your bike but it's well worth it in the long run.