Are there any practical differences between running a PRNG on a subset of bits from an RNG compared to running the RNG?
In 99% of cryptography there is no practical difference between your cases as long as $ |seed| = \frac{n}{2} \ge 128$.  Computational indistinguishability means that the streams from both cases will appear properly random. Our current knowledge is insufficient to invert a good well seeded CSPRNG (your PRNG). Information theory suggests that inversion can't be ruled out, but we can't do it as yet. Perhaps tomorrow or the day after. See P=NP and Kolmogorov/algorithmic complexity. 

The missing 1%.  For the truly dedicated who use one time pads (OTPs), there is a huge difference between the cases.  Information theory dictates that case 1 can create a OTP.  Case 2 specifically requires $ |bits| &lt; \frac{n}{2} $.  Otherwise the output is theoretically invertible. In this case though, the above theorem implies that the output from PRNG need only to not introduce bias.  It need not be cryptographically secure, nor even a dedicated PRNG. A simple random matrix will do.  But it begs the question of why bother if you already have a functional TRNG running at a similar rate?

Consequently, your RNG is more random than your  PRNG from a certain perspective.  It's the same differentiator that separates /dev/random from /dev/urandom in blocking *nixes.