You'll probably see lion in both. Could also see leopard and wild dogs, plus a whole lot more.
Selous has more open plains and lakes and rivers, so has more variety (and boat and walking safaris), but Ruaha is huge (Selous is just 'very big', at least the portion you may see) .
Selous and Ruaha have lots of brush, game tends to be skittish and not used to vehicles, so often you can't get as close as if in the north. It's also very hot and humid here and the air can be very still, hard to breath.
How many days to you have?
And, how would you plan to get to Selous/Ruaha? Road or air? Road is quite time consuming for covering both.
However, at this late date, it might already be difficult finding space in the north as it's peak season. But I still can't imagine that if your budget is too small for a camping safari in the north, it would be that much cheaper in the south, where it's also hi-season.
Maybe others have input for outfitter who can assist with the southern parks.
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