FEniCS will evaluate the first one at quadrature points within the element. You can verify this with the following quick test
from dolfin import *
mesh = UnitIntervalMesh(1)
V = FunctionSpace(mesh,"CG",1)
u = project(SpatialCoordinate(mesh)[0],V)
# One quadrature point in the middle of the element:
dx = dx(metadata={"quadrature_degree":0})
print(assemble((u**2)*dx))
which prints the midpoint evaluation of 0.25 when using one quadrature point at x = 0.5. The exact integral, without restricting quadrature, is 1/3, while interpolating at nodes first would give 0.5.
Note that nodal interpolation is used for Expressions in variational forms, e.g., continuing the example above with
expr = Expression("u*u",u=u,degree=1)
print(assemble(expr*dx(domain=mesh)))
prints 0.5, as expected from nodal interpolation of u^2 into a space of degree 1 (as indicated by the Expression constructor).