The Nokia X+ is a bumped up version of the Nokia X, but not by much, as only the RAM has been affected. It's up from 512MB to 768MB, and while it doesn't sound a lot, it makes a difference in terms of actual experience.
Switching between apps is a bit faster as is navigating around the operating system. You'll also be able to play a bit more demanding games too, which will otherwise be unplayable on the lower RAM on the X
The build quality and feel of the X+ is identical to the X and both have the same color options.
The bump in RAM increases the price of the X+ to &euro99. The Nokia X costs €10 less, so you'll have to decide if reaching deeper into your pockets for the X+ is worth it.