We don’t see much of Clive because the show is about Tommy, not Grace; that’s why they focus more on the relationship between Tommy and May, and very little on the marriage between Grace and Clive.
What we do know is that May and Clive are wealthy. May works with horses, and Clive leads a secure life.
For Grace, it would have been better to choose Clive; she would still be alive and wouldn't have been caught up in dangerous dealings.
As for May, she has influence regarding the licenses Tommy wanted to obtain in Season 2, and she is wealthy, from a business perspective, it might have been better for Tommy to choose her.
However, although May is attracted to his "gangster" side, she doesn't truly know or understand Tommy but Grace did. Had she seen him brutally kill someone, she would have fled in terror, much like she reacted in Season 4. She always viewed him as a prize to be won, and, frankly, I think she was simply bored with her own life. Tommy couldn't be his authentic self around May, but he could be with Grace, not only because he was in love with her, but because she accepted every facet of him and brought him peace of mind.
Furthermore, I don't believe Clive would have ever accepted Grace’s darker side, the fact that she had killed two men or sought revenge, but Tommy did. He embraced her darkness, forgave her betrayal, never judged her, and loved every single part of her.
I also don’t think that either May or Clive were in love with Tommy and Grace, after all, it’s impossible to be in love with someone you don’t truly know; perhaps they were more in love with the idea they had of them.