Provided they fit the child and install well both are safe. She may be thinking of some old Consumer Reports testing (I think the new TC tests are more about forward facing). I think they rated the Marathon poorly rear facing? Not positive. But then again, CR showed the Evenflo Discovery flying off of the base (it's since been recalled).
Either way, she'll need a convertible at some point, unless she gets a really really big infant seat and uses it to the limits and has a small kid and can last to three years or so in it. Other than the Discovery, no recalls have been issued because of CR or TC crash testing, so I don't stress too much about them. I do stress about internal decapitation and being 500% less safe forward facing, so if she's going to have a 22 pound infant seat and then go forward facing she's doing her child a huge disservice. Possibly a fatal one.
Have her come here to talk about her concerns. They're rather unfounded.
Wendy