While both Tomb Raider I and II are very similar games, with practically the same mechanics and graphics, I has a more claustrophobic, muted environment compared to II's larger colorful labyrinths.
While II bordered on nihilistic with it's prolific use of death traps - the original uses them sparingly.
Other than that, these are largely the same game, albeit the second with more outdoor contemporary locations - and a few items.
Overall, if you decide to go back, I'd recommend II over I.