the seville will be a little firmer than your deville is, but not a full on sports car; think luxury cruiser. The allante is more of a sports car; and expect to spend money on it. On the seville worst case it could nickle and dime you, and the biggest risk with that milage is the transmission, check the condition of the fluid. when it comes to the allante improper maintenance can cost an arm and a leg, and even the little things (like the odd sized bose that requires $150 speakers) can add up quick.
Another thought would be to look for a 92 seville or STS, or a 93 seville, those would be the next generation but with the 4.9.
Personally, I would love to find a nice 90-91 STS...that would be a fun little car