It would have to be a very lucky shot for a Plasma cannon to pop a Raider
I believe you would have to roll a 7 to glance!? But the theory is sound, blast the transports before they get close.
Unless the sentinels are making a real pain of themselves then there are not too many people who will choose to shoot at one over a Russ. But your theory is again sound, you could wall your russ behind some sentinels to give it some cover or protect its flanks.
Its what works for you...
Personally...
75 pts like the plasma sentinel = a Hydra
70 pts like the lascannon sent = Infantry squad with melta and autocannon
The 50 point scout sent with lascannon is the cheapest way to get a lascannon into your army (perhaps any army).
I suggest playing a few games with some proxy sentinels against several opponents and see if they work for you. Try out both the scout and the armoured and a few wep configs, even ones you think will be rubbish.
Then make your call.