"I beg your pardon" in a sense of appology: sumimasen すみません。スミマセン。"I'm sorry." gomennasai　ごめんなさい。ゴメンナサイ。"I beg your pardon." shitsurei shimashita　しつれいしました。シツレイシマシタ。"Appologies formy bad manner." "I beg your pardon" in a sense of "What did you say? I didn't get it.": mou ichido onegai shimasu　もういちど　おねがいします。モウイチド　オネガイシマス。"Once again, pelase." mou ichido itte kudasai　もういちど　いってください。モウイチド　イッテクダサイ。"Please say it again." "I beg your pardon." in a sense of "Excuse me, but..." shitsureidesuga　しつれいですが...　シツレイデスガ... "I'm sorry to bother you, but..." sumimasenga　すみませんが...　スミマセンガ... "Excuse me, but..." Katakana is usually used to write foreign words, but the corresponding sounds are same as Hiragana.

sumimasen or suimasen. it's almost same for hiragana and katakana. but we usually write it down with hiragana. so, if we write "sumimasen" in katakana, it's like struggling to say that. sometimes, we write "sumimasen" in katakana for an awkward moment.