Both departments are rated amongst the UK's top ten in the most recent Research Assessment Exercise for Electrical & Electronic Engineering. Both also received the highest marks possible for teaching quality.
Southampton has more academic staff and has a slightly higher RAE rating than Queens, but there is really little to choose between the two academically speaking. You should research into other factors, such as what it's like to live in Belfast and Southampton, what's the cost of living, what facilities do they of for international students, what are the course fees, etc. Good luck with your decision.