I'd say Seminyak & Sanur are chalk and cheese. Sanur is one of the original beach resorts and as a result gets a lot of the original 1970's larger Bali hotels. As a result it tends to get more groups (and has been nicknamed 'Snore' for the level of activity by some people!) and is used a lot by larger Tour Op's.
Seminyak has a couple of larger hotels, but tends to the top range but smaller hotels (Samaya, Oberoi, The Legian) and a whole bunch of smaller Pool Villa complexes like the Elysian, Amala, Villa Kubu behind them (some very good value). You won't (or - you might, but i'm not aware of!) the type of US$100 per room places in larger resorts you get at Sanur.
At nighttime it's very relaxed, and you find a lot of the uber trendy bars (Cafe Warison, Ku De Ta etc) are open. I like the vibe and feel of Seminyak, it's very relaxed place. Looks great at nighttime. Beach can be a bit 'surfy', but i've swam there a few times. As usual, lots of metered taxis, and easy to get everywhere else like Kuta when the need arises.
I havn't been to Sanur the last 2/3 times i've been though except to get off a boat from Nusa Lembongan, so not sure how much it has changed.