I'll say the Taiping Rebellion had more chances of success, because the An Lushan rebellion only got as far as it did due to some very bad moves by the Tang government.
Unlike the Taiping Rebellion, the An Lushan rebellion was not rooted in popular dislike of the ruling dynasty. When An Lushan first revolted, he put on the pretense of saving the Tang, by getting rid of Yang Guozhong. Quite a few officials and cities went over to An Lushan, but in many cases they resisted him when it was opportunistic. Before the Tang botched the defense of Chang'an and were forced to flee the capital, officials north of the Yellow River near An Lushan's home base would defect to and from An Lushan's side. Their loyalty to the Tang did waver, but it was never the case that they stuck idealistically to him or against Tang.
While the idea between Taiping Rebellion may been some Utopian Taiping Heavenly Kingdom, An Lushan's rebellion never stood for anything besides a pure power grab by one man. In contrast, the Taiping Rebellion had genuine anti-Qing sentiment.
Also, the Tang in the 750s were much stronger than the Qing in the 1850s. The Tang had their problems with corruption, but I would argue that they were less than the Qing at the time of the Taiping. Also, the Tang was still decently popular (see above), and the idea of the dynasty's legitimacy lasted. Note that even though there were regional governors who remained independent after the rebellion, they never declared their own dynasties. There was maybe one part of Tang China that had genuine antipathy towards the Tang government, but even that area didn't formally break away from the Tang even when they could.
If the Taiping had been more successful, they had maybe a small chance of taking over China Proper. In contrast, I would say An Lushan never had a chance in winning over all of Tang China.