First of all, the games do not use the same engine at all. That's a pretty ridiculous claim. Mass Effect uses the Unreal Engine 3, the same engine that powers Bioshock, Arkham Asylum and Gears of War. KotOR used the Odyssey engine, a modified version of the engine that was used for NWN1.
Secondly, Mass Effect is not just KotOR updated for the next generation, just because they are made by the same developer and are both Sci-Fi RPGs. The gameplay in combat is not the same, since KotOR uses a variant of the D&D 3e ruleset and is therefore a hybrid of turn-based and real-time combat based upon dice rolls, whereas Mass Effect is a 3rd-person shooter with skills merely affecting damage and accuracy, a bit like the system that was in Deus Ex. The gameplay out of combat, such as the questing and the openness of the world (i.e the freedom given to the player) is somewhat similar, but it's been the same in nearly all of Bioware's RPGs since the venerable days of Baldur's Gate. You might as well say that Mass Effect is an "updated form of Baldur's Gate" just because they're both RPGs in which you talk to people and travel around multiple quest hubs, but we all know that would be ludicrous and would require ignoring the myriad differences in gameplay styles.
Finally, Mass Effect's feel is different to KotOR's. Mass Effect was designed to feel more like an interactive blockbuster movie, since you do not choose Shepard's exact responses but merely point him/her in the direction you wish to take the conversation. KotOR is clearly much closer in style to the old Bioware RPGs such as Baldur's Gate series, but especially NWN.
I would suggest getting both, since both are classics in their own right. They're really not as similar as you seem to think they are. Anyway, all Bioware games deserve to be played, and you're shooting yourself in the foot if you disagree.