Out of the two new editions of both, I think Harrison's is much better. But unless you're a book junky like me, don't waste the extra money on getting both as Cecil's is still a good IM text.
You may want to check to see if your school's library offers Harrison's online for free - I end up using the online version much more often than my hard copy.
I would also suggest Step Up to Medicine as a great review text - nearly everyone at my school uses it (and our average medicine shelf scores are consistently at least 1 s.d. above the national average)
I never got a Harrison's. Similarly I only use my Robbins as a stand to elevate my laptop. What are these books really meant for in M1-M4? I feel like with the resources available on the internet these days they are becoming less and less applicable.