This is well tuff to answer so all i can do is be honest and let you decide.
Rfactor with mods/tracks and a full server is unbelievably good.
The mods being released at the mo are superb. The MMMV8's round Bathurst or Philip island are a dream come true. The McClaren mod round Laguna, Superb. The F3 Euros anywhere, superb. The Lienz festival mod again is superb. These mods especially the MMMV8's if anything have to much grip. You can chuck them about all over the place catching the back end and fishtailing up the road.
Great fun.
There are to many good things to say about Rfactor to write them here.
Now the bad.
Downloading the game and installing is a breeze but finding the right mod with the right track with people on can be a nightmare.
There are resource sites such as RSC and many others so if you decide to buy shout me and i will give you my list of sites etc.
You should be aware that as one mod may handle great the next may handle crap or you just might take to one more than the other. The variety on Rfactor is endless as mods are literally released weekly.
The cars handle brilliantly though the feel you get back through the wheel the noise the tracks the whole game is a true sim experience.
LFS.
I got this just after the 1.5 patch. I had tried the demo before but felt it lacking in feel. This is Not the case now. After trying the demo for an hour i decided to buy and i must say i haven't looked back since.
The cars feel excellent and are very instinctive which makes for lots of fun.
Servers have plenty of peeps on and there are no mismatch problems.
The LFS desktop feature is brilliant giving you immediate access to your own PB's and an upload feature for skins and many more excellent features.
Even when racing alone your PB's are logged which just adds to those lonely times on the track.
The new Saubers are unreal and skins are out so most F1 liverys are available.
I set a 56.03 at Blackwood today which i am pretty damn chuffed with seeings i've only had the game 2 weeks.
The initial interface is a bit dodgey and needs someone to point out the odd thing or two but apart from that it's a brilliant game.
So all i can say is i own both and race a lot, here is my simrace team site. I am [TSR]Wolf
http://www.topsimracers.net/Joomla/index.php
There is no one game that is better than the other in this case. I play both regularly. Today for instance i have played both going from one game to the next finding a full server with cars and tracks i want to race.
I would say buy both as i have.