He's worth the contract. I would give it to him, although he'll give up some taters. However, as some have pointed out, the real question is how much of a bat do you give up and how good (and how badly needed) is the SS.
If you have a decent SS AND the prospect is a .290-ish 25+ HR guy (and AA or above), then I might not make the move. If he's a A ball player then you have several years to replace the bat, all while you're enjoying the fruit of a very nice reliever.
Oh...if the SS can hit some that would influence it, too.
Perusing the lineups of both thems....I imagine that the SS you're looking at getting is Ramirez (his other AAA guys have no bat and two of them aren't SS's at all. Don't make the trade if those other AAA guys are being offered). Ramirez is going to be a fielder with 85-90 numbers across the board. maybe a point higher. He's a good glove..not great. But he's going to have nearly 70 power, close to 50/50 splits and not terrible contact and eye. He's a pretty good all around SS. My guys are better fielders..but not nearly the bat. he could probably survive at 2B or CF with the bat alone. He will have some nice seasons. And he's tough to find. I wish he were only 23, though. more years for you.
However, if your part of the trade is Alomar or Park, they aren't bad at all. Alomar is going to be a 70-70-70-70-90 type of player (est., of course, as I can't see the projections) and Park is a 60-80-90-75-90 guy. Both are decent. but both are 1B/DH's...and therefore are redundant. You could lose one easily.
Your Low A guy, Megias, is a better hitter yet. (And I'm betting he is the guy desired) He probably works out to be 75-100-75-75-80, or something close. Those are monster type numbers. 75 contact and 100-ish power is quite a combo. Your Jumbo Simmons is 69-91 and he hits nearly 40 homers a season (his vR is off the charts, though)
Megias is another level above Alomar and Park. HoF type of guy. He's only 19, too. He could be a MLB regular in 1.5 years. However, Alomar and park are only a year or two older.....and quite good. As well, bonking DH's can be found.
if the reliever and the SS can guarantee play-offs for you (maybe even make the difference between a first round exit and advance) then I would make the trade. If the guy that you have to give up is Alomar or Park, do it in a heartbeat. If you aren't a playoff team in the next couple of years, fuggid about it. Buy a reliever on the FA market (they'll show up) and dangle Alomar or Park and some other piece for a SS.
Even if it is Megias it isn't like you will be lacking big bats at DH and 1B with the other guys. But Megias along with one of the other guys playing 1B (Megias pegs as a DH better. his Stamina will mean some missed 1B starts) will be a terrific #3/#4 combo for a bunch of years, if you pay them.
but then, maybe I've guessed all the factors wrong. So ignore me in that case!
moe