It depends on several factors - What kind of music do you play? Do you play out in live situations or do you only play at home? Both amps are similar in some respects, but differ widely, too.
The Viceroy is small and loud at 7 or 33 watts. It's got loads of clean headroom and breaks up beautifully at higher volumes (think Mick Taylor, Billy Gibbons, Yardbirds-era Beck or Bluesbreakers-era Clapton). It also has a foot-switchable Mid-Boost for higher volume leads, a very nice reverb, and a Drive pot that subtly adds a little edge or grit to your primary tone. An inch and a half shorter than both the Rambler and the Mercury, it is our smallest amp in the 1x12 configuration. The Viceroy excels in all genres of music and is intended as a live amp; it might more amp than you need for strumming at home, especially with the Mid-Boost feature.
The Rambler is primarily a clean amp, but it has a great tone and handles pedals extremely well. It favors, in my opinion, Fender-type guitars, bringing out previously unheard depth and richness in single-coil pick-ups. The Rambler has limits to its headroom and the class A Cathode feel is a little more diffuse and yielding. It has a great reverb and tremolo built in. People that have never cared for tremolo find themselves seduced by it in the Rambler. The Rambler has a slightly warmer, softer clean tone that shimmers in crystalline fashion. It can be a great live amp or at home and is a favorite of blues players, jazzers, rockabillies, and country twangers.