The short answer is no; they vary slightly. Bulbapedia has all of May's responses, and I went through Emerald for all of Brendan’s.
Differences are bolded
After registering May/Brendan in the DexNav:
"MAY: Hi, [player]! MR. BRINEY retired as a SAILOR, but I still see him out on the sea with his pet PEEKO sometimes. He must love the sea still."
“BRENDAN: Hey, [player]! MR. BRINEY retired as a SAILOR, but I stilll see him out on the sea with his pet PEEKO sometimes. I guess he must love the sea still.”
After receiving the Knuckle Badge:
"MAY: Hi, [player]! You know how little towns like PETALBURG and DEWFORD have GYMS? For some reason, the big port of SLATEPORT doesn't have a GYM. When they finally build a GYM there, I should apply to be the LEADER."
“BRENDAN: Hey, [player]! I don’t get how little towns like PETALBURG and DEWFORD have GYMS. I mean, not when the big port of SLATEPORT doesn’t have a GYM. When they finally build a GYM there, I should apply to be the LEADER.”
After delivering the Devon Goods:
"MAY: Hi, [player]! Do you remember a man named the CUTTER in RUSTBORO? He had a house there, remember? Well, I found out his younger brother lives in MAUVILLE. Can you guess his name?
… … … … … … … … … … … …
The ROCK SMASH GUY!"
“BRENDAN: Yo, [player]! Do you remember a guy named CUTTER in RUSTBORO? He had a house there, right? Well, it turns out his little brother lives in MAUVILLE. Can you guess his name?
… … … … … … … … … … … …
The ROCK SMASH GUY!”
After defeating Wally in Mauville City:
"MAY: [player]? RUSTURF TUNNEL…They named it that because it joins RUSTBORO and VERDANTURF."
“BRENDAN: This voice… [player]? They gave RUSTURF TUNNEL its name because it joins RUSTBORO and VERDANTURF. Did you know that?
After connecting the two halves of Rusturf Tunnel:
"MAY: [player], how are you? I'm out on ROUTE 111 now. I'm going to get a rest at an old lady's house. She lives just north of the desert."
“BRENDAN: [player], what’s up? Hey, I’m out on ROUTE 111 now. I’m going to rest up at an old lady’s house north of the desert. If you’re in the area, you should visit her, too.
After receiving the Heat Badge:
"MAY: Hi, [player]! Did you know about the MIRAGE TOWER in the desert? They say, like a mirage, it seems to mysteriously come and go. I wish I could see it.”
“BRENDAN: Hey, [player]! Did you know about the MIRAGE TOWER in the desert? They say, like a mirage, it can be seen only sometimes. I’d like to see that!”
After receiving the Balance Badge:
"MAY: [player], yahoo! I'm on ROUTE 119 now. There's a big river here, and it often rains. I got soaked!"
“BRENDAN: Who’s this? Oh, [player]? Guess what? I’m on ROUTE 119 now. There’s a big river here, and it rains all the time. I got soaked to the bone!
After defeating Shelley at the Weather Institute:
"MAY: [player], hi. MT. PYRE is a memorial to POKEMON whose lives have ended. Maybe as a result, it's infested with many GHOST-type POKEMON!"
“BRENDAN: [player]! MT. PYRE is a memorial to POKEMON whose lives have ended. That’s probably why it’s infested with many GHOST-type POKEMON!”
After defeating Team Aqua at Mt. Pyre:
"MAY: Hi, [player]! I was thinking of going to the hot springs in LAVARIDGE. But on the way, around JAGGED PASS, I ran into some bad-looking characters. The mood was ugly!”
“BRENDAN: Hey there, [player]. I was on my way back to the hot springs in LAVARIDGE. But around JAGGED PASS, I ran into some nasty-looking characters. Those creeps… I think they were TEAM MAGMA.”
After defeating Maxie at the Magma hideout:
"MAY: Hi, [player]! Did you see the news? They say CAPT. STERN discovered the SEAFLOOR CAVERN while on his submarine expedition."
“BRENDAN: Hi, [player]! Did you catch the news? They say CAPT. STERN discovered the SEAFLOOR CAVERN while on his submarine expedition.”
After watching Archie take Capt. Stern's submarine:
"MAY: Hi, [player]! Don't you think it's neat? Even if you don't have a boat, you can cross the sea using a POKEMON's move. Did you know there's more? There's a POKEMON move that lets you go to the bottom of the sea."
“BRENDAN: Hey there, [player]! Don’t you think it’s awesome? Even if you don’t have a boat, you can cross the sea using a POKEMON’s move. And, there’s a move that lets you travel to the bottom of the sea. Man, POKEMON can do anything!”
After defeating Matt at the Aqua Hideout:
"MAY: Hi, [player]! Hope things are okay! Have you been on the sea and found your way to the other side blocked? Try diving deep underwater and following trenches. When you get to the other side, come up to the surface. Easy!"
“BRENDAN: Howdy, [player]! How are you holding up? Ever found your way to the other side blocked while on the sea? Try diving deep underwater and following trenches at the bottom. When you get to the other side, come up to the surface. Simple!”
After meeting Steven on Route 128:
"MAY: Hi, [player]! How's it going? Are you filling your POKEDEX? I heard a rumor that there are super-ancient POKEMON out there. And there are three of them! I would love to see even one..."
“BRENDAN: Hey there, [player]! How’s it going? Filling up your POKEDEX successfully? I heard a rumor that there are super-ancient POKEMON out there. And not just one—three! I’d love to catch even one...”
After receiving the Rain Badge:
"MAY: [player]! I heard the rumors! You beat the SOOTOPOLIS GYM LEADER? That means you don't have far to go, do you?”
“BRENDAN: [player]! I heard the rumors! You beat the SOOTOPOLIS GYM LEADER? Awesome! You’re getting awful close now!”
After becoming the Champion:
"MAY: There isn't a single TRAINER left in HOENN who doesn't know who you are, [player]! When I tell people that I'm friends with you, [player], they're all surprised!"
"BRENDAN: There isn't a TRAINER in all of HOENN who doesn't know who you are, [player]! When I tell people that I'm friends with you, [player], they get pretty envious!"
