I was going to reply to this last night but ... I'm not sure if this thread is breaking forum rules or not. I mean, it is definitely against the '
indecisiveness' rule, where you're not allowed to create threads to ask people to make up your mind for you, but that rule may only count for the bokumono forums, to keep people from continually asking which bachelor/ette they should choose. I'm going to answer based on benefit of the doubt but if this is against the rules, I'm sorry for contributing to rule-breaking. (EDIT: Looked it up again after posting and realized I probably am since it specifically mentions asking about what you should purchase. :/ Sorry!)
You're going to need someone who has played both games to be able to get good reasons for why to pick one over the other. I'm afraid I can't help. I LOVE Xenoblade...it's one of my all-time favourite games...I loved the story and the world and the characters...and
the music, my goodness, never heard such an amazing soundtrack on a 3DS game before (every region has
its own song, totally
different from the
others, and each has both a day version and a night version)...but I can't give you a comparison to how great it is versus Radiant Historia, as I have never played that game. Personally, I don't like Radiant Historia's graphics all that much, not even in the remake, which for me is a major hurdle at the very start and has kept me from picking it up even though I hear the story is great, probably comparable to Xenoblade's lengthy and detailed story, but it all boils down to personal preferences.
I also like to save the best for last in everything I do so if I could only have one game at a time, I might well end up leaving the game I wanted most to be the one I got second.

One thing to keep in mind is that Xenoblade is a LONG game. I think I spent nearly all my time on it for a month before I finished it. We're talking 150 hours of playtime or more, my first time through (I did ALL of the side quests, and there are about 450 of them...not kidding!...though some are mutually exclusive. Thankfully there are very good guides online if you're the type who wants to find every quest...many of them feel like they add to the story/world, so I love 'em.)