Jennifer Hudson is more well-known and arguably more successful because she has one big movie, more awards and a higher status.
But Jordin Sparks is obviously the bigger star in Pop music considering Hudson has exactly 0 hit. And Sparks has hits like "Tattoo" and "Battlefield" (And "S.O.S. (Let The Music Play)" went #1 on Dance, Bulgaria and Top 40 on Pop radio -- More than what can be said to any Hudson single; she also sang the backup vocal on Demi Lovato's "Skyscraper", a Hot 100 Top 10 hit) even though after her debut era, her albums sales is no better than Hudson's. For radio and Pop music fans as a whole, Sparks is definitely bigger, but Hudson has a stronger / bigger fanbase because of her 'celebrity' status. Just think about this -- even the disastrous "I Am Woman" got more attention than any Hudson singles. Think currently Rihanna vs Lady GaGa.
And with Whitney Houston's sudden and sad passing, "Sparkle" could very well bring a big comeback for Sparks, and mark her big breakthrough if she gets the right materials (Both the aforementioned "Skyscraper", Rihanna's "Complicated" and Selena Gomez & the Scene's "Who Says" were offered to Sparks alongside IAW -- Even though she seems to be only choosing the crappier tracks like IAW, "The World I Know" and that forgotten new song released somewhere) and times things right. I mean, even Orianthi got a breakthrough with "According To You" following Michael Jackson's passing. Deaths have been known for boosting careers, like Princess Diana to Sir Elton John and P. Diddy, Lil Wayne, Danny Gokey, Janet Jackson and of course Hudson herself.
And by the way, Houston has always been a pretty successful movie star, with movies such as "The Bodyguard", "Waiting To Exhale" and "The Preacher's Wife". I think "Sparkle" will actually be her first movie role after TPW and "I Look To You" debuted #1 on the Billboard 200 with more than 300k copies of sales (Blocking Miley Cyrus' "The Time of Our Lives" EP -- Someone who has had 4 straight #1 albums and 2 other Top 3 albums from her debut, and had blocked the likes of Kelly Clarkson and Sugarland back then) and scanned passed Platinum. The single "Million Dollar Bill" also went #1 on Dance. Granted, the singles did not do well on most radio formats or in sales but there have been very few 40 + women who are able to score mainstream hits, with the exceptions of only Madonna, Cher, and perhaps the very current Jennifer Lopez who scored only one with the assistance of American Idol and Pitbull. Even artists like Mariah Carey failed (Her last hit, "Obsessed", was released when she was only 39). Her tour got poor reviews and receptions, but the tickets did sell well. I'd hardly say she's been crashed and burnt before her death.