There has never been a spring like it for cultural openings and it's not over yet: more museums and galleries are on the way. We need them, if only to divert some of the millions of visitors from the frenziedly popular new Tate. But for now, let's explore the David-and-Goliath nature of this week's pairing. Dulwich is relatively tiny and was the nation's first purpose-designed public art gallery, by Sir John Soane in 1814. Restoring it and adding some modest extra space has cost £8.3 million. Somerset House is a sprawling, multi-level palace of civil servants, dating from the 1780s, designed by Sir William Chambers: it is very good, but not nearly a masterpiece. Restoring it and opening it up to the public has cost £48m. Dulwich is a perfect little building with a fine collection and a strong reputation: Somerset House is a flawed big building with an untested brand-new collection, no history of unfettered public access and - thus far - no reputation at all.

So while the Dulwich Picture Gallery will be welcomed back after its closure like an old friend, its familiar contents from Van Dyck to Claude now freshly displayed, Somerset House has all its work to do. It has, for instance, to try to make us love the Gilbert Collection, itself costing at least £21m to house, which occupies a large part of the southern ranges of the building. This is a multi-millionaire's hoard of exceptionally valuable gewgaws, from jewel-encrusted snuffboxes to silver-gilt gates from Russian Orthodox churches via some thankfully rare Italian "micromosaics" - kitschy attempts to make framed painting-like things out of inlaid stone. Sir Arthur Gilbert's collection - he is a London-born, Californian-based real estate developer in his late eighties - is on a scale to compare with London's Wallace Collection, gifted to the nation a century ago, which also has some differently iffy bits. But whether or not you like this particular kind of over-ornate treasure, it has been well displayed here.