There is no question that the Queen's Gambit gets much more respect at the higher levels. However, the best for you depends a lot on your personal circumstances. Here are some books to help, either way:
A Strategic Chess Opening Repertoire for White by John Watson (2012). Playing 1.d4 by Lars Schandorff (2012). The Kaufman Repertoire for Black & White by Larry Kaufman (2012).
https://web.archive.org/web/20140627105428/http://www.chesscafe.com/text/hansen161.pdf
https://web.archive.org/web/20140626221508/http://www.chesscafe.com/text/hansen162.pdf
Win with the London System by Sverre Johnsen & Vlatko Kovacevic (2005)
https://web.archive.org/web/20140627074459/http://www.chesscafe.com/text/hansen85.pdf
Play the London System by Cyrus Lakdawala (2010)
https://web.archive.org/web/20140627100246/http://www.chesscafe.com/text/hansen139.pdf
The November 2015 issue of Chess lists the top twenty openings compiled from a list of 1452 September games where both players were rated over 2400 Elo. One can not take position on this list too seriously because it is greatly influenced by how the openings are grouped. For example, all the Retis are grouped together, while English is separated into 1 ... c5, 1 ... e5, etc. Nevertheless, for what it is worth, the list reports 71 Slavs, 61 Caro Kanns, 56 Najdorf Sicilians, 56 Declined Queen's Gambits, 55 King's Indians, 49 Ruy Lopez Berlin Defences, 46 Nimzo-Indians, 40 1 ... c5 Englishes, 39 Queen's Indians, 37 1 ... e5 Englishes, 37 Gruenfelds, 31 1 ... Nf6 Englishes, 26 Kan Sicilians, 25 1 ... e6 Englishes, 24 2 Nf3 sideline Sicilians, and 24 Taimanov Sicilians.
i have been studying both of them for quite of a time. what do you think is better? the London System or Queen s' Gambit? i have Positional/Solid Style.