I think it all depends on WHERE & HOW you're planning to fish. If you're doing more than 2 nights, with all that food, bait, extra clothing & not feeling too clostrophobic then I think a 2 man is best. You can set everything well back (including your bedchair) & you've still got all that porch space, which means easier movement plus extra room for a carpet, your stove, bivvy table & even a guest or 2 for socializing. But yeh, for a night or 2 a simple brolly system is ideal, especially if you plan to move after spotting fish 100 yards down the bank.
Mind you, all the new 2 man bivvies are just as easy to set up & put down as the 1 mans, if you choose the right one.