With regard to the on-train experience there is practically no difference between "Leisure Select" and "Business Premier". The seat is the same and although they are normally segregated by carriage (so all LS in one and BPs in another).

The meal is also the same, with the exception being for morning trains where BP passengers are offered an "express" option on breakfast so the tray can be cleared quickly so they can continue working.

The Lounge at St Pancras is quite nice and I'm going to disagree with Wong Jnr and say it is bigger than Waterloo. The trick here is that most people don't notice the lounge is on 2 levels so only stay on the ground floor and don't head up to the upper floor which is usually very quiet and much nicer!

BP also officially comes with 10 minute check-in but anecdotal evidence from friends/family/colleagues/minor acquaintances suggest that the 30 minute(?) rule for others is not that strictly enforced.

So it basically boils down to lounge access. At St Pancras the general departure area is quite nice, has a Café Nero etc so is quite a pleasant place to wait. Paris however is pretty awful, usually overcrowded and you'd want to spend as little time as possible in the general departure area. You may therefore want to consider just upgrading to BP one-way on the return. Though this is likely to be at least £50 per ticket so perhaps quite expensive for the limited waiting time.