I'd say the original, if only because it's only $10 on PSN and Twin Snakes is most likely difficult to find. But other than that you can't go wrong with either, though they have some relatively minor differences that may influence your decision:
For one the music is definitely better in the original, although Twin Snakes soundtrack isn't terrible. The voice acting was also completely redone for the Twin Snakes, most notably with Naomi and Mei Ling losing their accents (They also no longer have their accents in MGS4 if that counts for anything) and some various lines having been cut and some new added, but again nothing too major. Twin Snakes also has some added gameplay features from MGS2 which give you more options, but things like the tranq gun and first person aiming can make the game significantly easier should you choose to use them. Also the Twin Snakes has new cutscenes which many fans found too over the top, (Snake back-flips over bullets and jumps off an incoming missile) although after playing the other games in the series I'd say that's fairly tame in the crazy department. Lastly Twin Snakes has better graphics, sound quality etc. and slightly refined controls.
Oh, and the Twin Snakes doesn't have the VR missions from the original, hope that all helps!