Actually, now that you mention it, they ARE very similar in singing. Christina's songs contain more inflections (ups and downs), which makes her sound like she's got a great range, which she does. But if Beyonce sang the same songs, you would not dim her incompetent. And Christina does this thing with her voice that makes it all very weird. She's definitely a vocal belter, but she does this belting all the time and it's so weird. It's creative and nice at times, but then it's kind of weird.
Beyonce is neither extremely original nor seemingly original, but she doesn't need nobody to fix her songs after she's done recording! =D
I think they're both great singers, and I love them both very much. Beyonce dances way better, though.
IN A NUTSHELL: For the creative stuff, Christina wins. For everything else, Beyonce is better I think.

Album: "Dangerously In Love" (2003)

Crazy In Love
Naughty Girl
Baby Boy
Hip Hop Star
Be With You
Me, Myself & I
Yes
Signs
Speechless
That's How You Like It
Closer I Get To You
Dangerously In Love 2
Beyonce Interlude
Gift From Virgo
'03 Bonnie & Clyde
Work It Out
Daddy

soundtrack: "The Fighting Temptations" (2003)

Fighting Temptations
Fever
Everything I Do
Swing Low, Sweet Chariot
He Still Loves Me
Time To Come Home
Summertime (w/ P. Diddy)

album: "B'day" (2006)

Déjà Vu
Get Me Bodied
Suga Mama
Upgrade U
Ring The Alarm
Kitty Kat
Freakum Dress
Green Light
Irreplaceable
Resentment
Creole [Japan bonus track]
Check On It
Lost Yo Mind [iTunes Pre-order Exclusive]
Encore For The Fans [Bonus Track]
Listen [Hidden Track]
Get Me Bodied (Extended Remix) [Bonus Track]

album: "B'day Deluxe Edition" (2007)

Beautiful Liar
Irreplaceable
Green Light
Kitty Kat
Welcome To Hollywood
Upgrade U
Flaws And All
If [EU/Asian/Australian Edition]
Still In Love (Kissing You)
Get Me Bodied
Freakum Dress
Suga Mama
Déjà Vu
Ring The Alarm
Resentment
Listen [Hidden Track]
World Wide Women [Hidden Track]
Amor Gitano
Oye [Listen – Spanish Version]
Irreemplazable [Irreplaceable – Spanish Version]
Bello Embustero [Beautiful Liar – Spanish Version]
Beutiful Liar (Remix)
Beautiful Liar (Spanglish Version)
Irreemplazable (Nortena Remix)

album: "I Am… Sasha Fierce (Deluxe Edition)" (2008)

If I Were A Boy
Halo
Disappear
Broken-Hearted Girl
Ave Maria
Smash Into You
Satellites
That's Why You're Beautiful
Save The Hero [Japan Bonus Track]
Si Yo Fuera Un Chico [Spain iTunes Bonus Track]
Single Ladies (Put A Ring On It)
Radio
Diva
Sweet Dreams
Video Phone
Hello
Ego
Scared Of Lonely
Why Don't You Love Me? [Japan Bonus Track]

other songs:

A Woman Like Me [from "Pink Panther" soundtrack]
After All Is Said And Done [from "The Best Man" soundtrack]
Cards Never Lie [from "MTV's Hip Hopera: Carmen" soundtrack]
Have Your Way [from "His Woman, His Wife" soundtrack]
If Looks Could Kill (You Would Be Dead) [from "MTV's Hip Hopera: Carmen" soundtrack]
I'm Leaving
In Da Club
Keep Giving Your Love To Me [from "Bad Boys II" soundtrack]
Naughty Girl (Remix)
Stop That! [from "MTV's Hip Hopera: Carmen" soundtrack]
Summertime
Summertime (Remix)
The Last Great Seduction [from "MTV's Hip Hopera: Carmen" soundtrack]
Wishing On A Star [from "Roll Bounce" soundtrack]

THE ONLY SAFE way to learn the correct singing techniques & to improve properly IS to take OFFLINE face-to-face singing lessons with a fully trained vocal teacher (in case you do not have a teacher yet)! The teacher HAS TO BE in the same room with you, so that he/she could give you proper feedback.

Please do NOT rely on any dodgy web tutorials because that way you can misunderstand things VERY EASILY and develop bad habits, hoarseness, vocal nodules and other nasties IN NO TIME, and even though you would sound good! It is always much wiser to invest a little bit of your money/time to face-to-face lessons rather than wasting the same amount of money (or even more!) to frequent ear-nose-throat specialist visits due to aforementioned problems, so please reconsider this. If you can't afford vocal lessons, then joining a choir is the only SAFE alternative option. And believe me, but even MANY of those who have music as their hobby DO take lessons as well!

Always remember to warm up your voice properly, but please know your limits and don't overdo your voice! Remember the diaphragmatic support, do not strain your throat too much! Also, remember good body posture!

Avoid fizzy drinks (burp danger), dairy products (mucus risk), caffeinated products (coffee & tea included, they dry up your throat) and spicy food (irritation risk)! You can consume these things, but NEVER before singing!

Do NOT shout, yell, scream nor otherwise abuse your voice AT ANY TIME! Also, please respect your vocal range; if your teacher says you are, say, more of an alto (baritone if you are male), then you ARE more of an alto (baritone).

DO NOT try to imitate anyone famous, that will usually give you just bad habits and even damage your throat. You are YOU and your voice is unique, so please learn to cherish that. The world does NOT need copycats.

Do NOT sing, whisper, shout, yell nor scream if having a sore throat/cold/flu, Also, do speak as little as you can if you have flu/cold/sore throat!

Remember to drink at least 2 litres of room-temperature still water every day, not just during singing days!

Smoking is a big no-no, as is inhaling secondhand smoke. Also, avoid staying in dusty and/or moldy environment.