I adore them both. I mean, really adore. The characters, the plots, the period. I suppose Giordano Bruno is the sexier lead, but I slightly spoilt my enjoyment of him by looking him up (he was a real life character). Knowing his end casts a shadow in my mind every time I am in his company.
Shardlake is another blighted soul but at least he is entirely fictional so Sansom can go anywhere with him. I never knew lawyers could be so attractive, but find myself oddly drawn to the sexual power behind a man who can argue his way out of any situation. And he has the advantage of the adorable Jack Barak (book 2 onwards) as his sidekick.
I would say that Sansom is the superior mind but neither series is hard to follow. On the contrary, they are genuine page turners, always lively and interesting, occasionally funny, loads of little cliff hangers you can't resist even though it is way past your bedtime or you have a list of things to do.
So, in short, read both, and gosh am I jealous of you for being at the start of such an enjoyable reading list.