Ok I have time, I'll run the numbers for you.
Here's how the units stack up:
Nurglings are absolutely smashed by templates, all basic troops (except Tau Fire Warriors and Grots, an elite bunch to be grouped with!) beat them in
CC, and you have to have a way to get the tally up.
I mean, you don't even have Noxious Touch on your demon princes. Do you expect Plaguebearers to do it? Just Ku'Gath? Mr. Run Away or Pound With Plasmas himself? Seriously, this doesn't work. At 2500 points I can remove him in a single turn of shooting. 2500 points I can also assault him to death in a turn. Really. The game is that different at 2500 points.
So, back to the analysis of nurglings vs beasts of nurgle.
Close combat:
Beasts of Nurgle are WS3. So except against less than a dozen enemies in the entire game, you hit EVERYONE on 4+ and crappy troops like
IG don't hit you on 3+.
Nurglings are WS2. They have serious issues hitting elite combat troops (that's anyone with WS5 or better). You also get hit by crappy troops like
IG on a 3+.
Nurglings thus are not as good in
CC.
Shooting/close combat:
In shooting, everyone wounds you on 4+ (lasrifles? really? wow you suck!) or more likely 3+ (bolters, flamers) and 2+ for S5 and S6 weapons.
Beasts on the other hand, require 6+ for lasrifles, 5+ for bolters, 4+ for S5, 3+ for S6...I mean, you do see it is much harder to torrent away Beasts because T5 is always far superior to T3?
Saves:
Nurglings take multiple wounds from templates (yes, I know they have improved cover saves, but since flamers ignore those...uhhh neat, and if you are out in the open indirect fire is also crippling. face a 15 mortar
IG army, you'll feel the pain). They also do not have
FNP.
Beasts of Nurgle DO have
FNP. Which tally improves.
Wounding/Attacks:
Nurgling bases have poisoned attacks? They do? What page is this on? Right, only if the tally hits 10+, which in your army will take forever since you cannot kill anything with your build.
Nurglings are base strength 3, so they don't do damage like beasts do. Even having more total attacks doesn't impress anybody.
Beasts can get noxious touch on one "hero", so they wound anything on a 2+ (so
MC don't just smush them like they do nurglings) which really does help.
Beasts also have S4, and that means alot. Especially against elite troops (think Khorne zerkers or grey knights or death company...who rip nurglings up but have a tough time wounding beasts...where beasts do NOT have a problem wounding them).
Nurglings can kill guard, and beasts kill them just the same. Nurglings do not kill marines, but beasts do.
Numbers:
Nurglings have 4 attacks on the charge.
So do beasts (on average).
Nurglings get shot to pieces and chopped to pieces by almost anything and anyone.
Beasts die to high strength AP1 and AP2 weaponry (denies
FNP) and high strength (or rending) power weapon hits in
CC (also denies
FNP), but will stick around against regular attacks from any basic troop in the game where nurglings get attritioned to death.
See, when you lose 10 wounds in a nurgling unit and only cause 2 wounds, you take 8 fearless wounds. This is the price you pay for being

units. Then you lose more nurgling swarms.
Beasts, don't lose combat by anywhere near that amount because only power weapon (or rending) hits bring them down. Regular attacks? T5, 5+, 4+. Yeah, go ahead and swing, tactical marine. Beasts will laugh at you.
Oh and
fyi, Beasts give you support to your
MC by providing them with a 4+ cover save. Swarms cannot, and thus are crap.
Plaguebearers beat no one in combat, not until tally 20+. You do realize the games probably over by the time you scrape your way to this number with this list?
A better army list:
I'll post that separately so you can tell me which you think really is "better".
Here is a link:
http://www.dakkadakka.com/dakkaforum/posts/list/0/216021.page