As long as its the smaller calibre it doesn't matter too much which 400 you buy to be honest.
But I would recommend the S400 for any new starter. Carbine or standard.
I shoot an MPR in HFT and own a couple of 400's and a 410, and they are all capable of being an excellent all round gun.
If you do need a silencer though it might be easier to not buy the MPR, as the standard silencer does not fit staright on and you will need an adaptor. I keep my MPR purely for HFT and shoot a 400 or 410 for hunting.
Oh and the MPR has a few more subtle changes than you might think, as it has a bigger diameter match barrel and a different breach, with a dedicated .177 loading channel, adjustable trigger blade and a stock that I dont like to be honest, and have found uncomfortable so I dont use the MPR stock. Although it has bog standard and simple 400 internals which is a bonus in my opinion.
Hope this helps a little, although as has been said in the posts above, try a few guns out and dont rush to buy unless it feels comfortable.