I'm currently a 2nd Classman at VMI (or Junior), so I feel like my experience is applicable.
I'm not going to sugarcoat it-the Ratline is tough. Short nights, long days, rigorous exercise, and a lot of stress-and that's just being a Rat without academics thrown into the mix. Life at VMI is about as spartan as you can get it at a collegiate institution. You have constant inspections (room and uniform), the rooms are essentially the bare necessities, and the academics are difficult, especially if you're a STEM Major. Additionally, VMI currently is the only SMC to maintain a true single sanction Honor Code in the United States at this point.
As for ROTC, the Blue Ridge Battalion is the largest Army ROTC program in the US, and the instructors genuinely care about your success. ROTC has been a great experience for me, and I was lucky enough to pick up a 2-Year scholarship at the beginning of the semester. The contracts are there, you just have to work hard for them.
However, there are the lighter moments. Throughout the Ratline, and across your 4 years here, you'll make bonds that you'll carry for the rest of your life. Life at the Institute is stressful, hard work, and quite frankly sometimes exhausting-but the men and women around you make it worth it. I have friends here that are more like my brothers, and I can say with absolute certainty I'd do just about anything for them.
Either way, if you have to choose between the two, they're both great choices. I've got a lot of respect for my counterparts at The Citadel, even if I am a just a bit biased towards Mother I. As for some differences, I'm 90% sure that Knobs don't have to keep their heads shaved the entire year now-Rats do. The Citadel is in Charleston, a very beautiful college town. VMI is in the middle of nowhere in the Shenandoah Valley. Other than that, I don't really know much, but I'm sure someone who goes/graduated there can help you out on that.
Hope this helps and if you have any questions feel free to PM me and I'll see if I can help!