I like Declan because it would make a nice, subtly Irish-themed match for Rowan, and the ending -n ties them together. On the other hand, Silas matches well because they have the same number of letters and syllables, and it matches the soft but masculine sound of Rowan (while Declan is a little "harder" sounding than Rowan because of the D and C). Silas and Rowan would also be a good match for brothers because both are pretty distinctive. Miles is fine but I wouldn't pair them for some reason, I picture a Miles and a Rowan as looking very different and so can't really see them as brothers. Miles would be a great middle name for Declan though.
So, I would probably plan to use Silas. All three are great though, so I'd keep Declan and Miles in mind just in case he doesn't look like a Silas.
Me , DH , Emmy (5/08), and new baby Tilda (1/10)