For what it's worth I'd go for a TX. If you buy an S400 and really get the bug then you would probably want to buy a full-blown match rifle.
If this happens then the S400 will become totally redundant. If you buy a TX and go on to buy a match rifle then you might well want to keep the TX for spring gun comps.
Look at it this way, the TX is one of the very best spring FT rifles you can get whereas the S400, while a superb rifle, is not one of the best FT PCP rifles you can buy.
Having said this you can't go far wrong with either.
Good luck whatever you do, and I hope you get as much enjoyment from the sport as I do!