Both have their pros and cons.

Cruiseplum custom search can query for solo fares and has a solo supplement query. This is the easiest way to check for Quantum and Anthem of the Seas studio cabins which I've found so far. Query performance is good as is sort performance. The view is clean and the price, port, itinerary information is there along with a direct booking link to that specific itinerary. I wish the custom search showed more search results in one page but perhaps my queries are broader than most users.

VacationsToGo has a larger database and other kinds of useful queries good for specific itinerary searches. The custom search doesn't allow for solo price queries, though. Query performance is also tolerable but the larger database means sometimes the result set is too large to display. Results are always broken up by destination and then a column based sort. Color highlight on the first and last column is useful but gives me eye twitches. You can't pull up actual prices for all sub-categories so if you don't want to book a base cabin (or have a specific cabin class in mind), you have to call VacationsToGo, check the cruiseline's website, or start price comparison with other cruise agents which do show all available cabin prices.