Marty breathed life into both Telly and Snuffy as far as I'm concerned. Both characters were pretty much one-dimensional in the hands of their original performers: Brian's Telly was always whining and bellyaching over something, and Jerry's Snuffy was a giant Eeyore. Marty brought a broader emotional range to both characters that they lacked beforehand: his Telly is so lovably neurotic, and can get passionate, highstrung, and manic about anything; and I honestly can't imagine Jerry's Snuffy singing and dancing to such a bouncy and upbeat tune like "Bubble on my Snuffle."
It's still interesting to see the transitional period though, and I think Marty is a prime example of a character inheritor, who tried a little too hard to basically imitate the way his predesessors did the characters he inherited, before making the characters his own. FTB is a good showcase of Marty doing a slightly whiney Telly in a similar vein as Brian, and his Snuffy voice is softer and less emotive, a la Jerry.